AB/ABY/ABC channel switchers
Imagine that you have two amps that you would like to switch between for different sounds. Or that you have two pedalboards with different settings that you would like to switch between without having to unplug any cables. Then an AB, ABY or an ABC switch would come in handy for you. The difference between them is how many channels they can switch between - AB can switch between two channels, ABY can switch between two channels but also run them together, ABC switch is like an ABY switch but with three channels instead of two. Many of these units can also be used the other way around, hook up two or three guitars (or one guitar with two different output jacks) to them and change channel when you switch guitar. Handy pedals, right? There are many choices, I myself own the Morley George Lynch Tripler which allows me to switch between three different amps (or using them simultaneously, which sometimes sounds great). Other options includes the Voodoo Lab Amp Selector and a whole line of pedals from Lehle. Make sure you get a pedal that doesn't affect your tone, as some of the channel switcher pedals on the market tends to steal some of your thunder. However, those pedals mentioned here won't dry you out, they are premium quality.
Of course you'll need a tuner because no matter how many great pedals you own, a bad tuned guitar ruins them all. There are some good pointers when you choose your tuner: make sure it's accurate, that the light on its display will work out on a dark stage, that it kills the output tone when engaged (you don't want the audience to hear you tune your guitar) and that it won't take up more space than necessary on your pedal board. Some great choices are the Korg Pitchblack, the TC Electronic Polytune or the Boss TU-3.
There's some simulators on the market that makes your guitar sound like another instrument. To start off there's acoustic simulators that makes your electric guitar fake that it's an acoustic one. Many guitarists uses piezo microphones on their guitars for this simulation but there's also some pedals out on the market. Of course they won't sound exactly like an acoustic guitar, that would be too good to be true, but they can come pretty close sometimes. Which may be just what you need if you've only got one acoustic song on you setlist but haven't got the possibility to bring an actual acoustic guitar. Then the Boss AC-3 or a Mooer Acoustikar could be your saviour. My experience with units like these is that you'll get the most true acoustic sound when you use the Line Out jack straight to a PA system. Just like a real acoustic guitar, these pedals doesn't do themselves justice when plugged into an amp (unless it's an amp especially built for acoustic guitars). Other simulators includes organs (Electro Harmonix B9 and C9) and sitars (Electro Harmonix Ravish Sitar) and surely there are there are some other cool units out there. Just remember, these pedals are just simulators, they can never be the real deal. But they can both be fun an useful, and they could be just what you need.
How many times have you played a song in a band and when it't time for your guitar solo nobody hears it? That's probably because you've started the song at the highest volume on your instrument and when you need a push to cut through the mix you have none. Sure, you can use the volume control on your guitar, but this control often gives your instrument different tones depending on where it's set, and chances are that you like your tone to stay the same all through the song. Then a boost pedal will come in handy. Activate it and the volume of your guitar will raise without changing your tone. When your solo is done, just de-activate the boost pedal and you will get back to the volume which sits in fine with the rest of the band. Some boost units have controls for gain as well, or some other features, which can be great if you need it. They can sometimes also be used as a preamp to boost an amp into overdrive, Brian May of Queen is one guitarist that uses a booster (built by Roger Taylor, the bass playerof Queen) this way. Some options are the Xotic EP Booster, the Boss FB-2 (which also has a simulated feedback effect), the RC Booster, the Visual Sound True Tone or the Suhr Koko Boost.
Here's another pedal which feature can be achieved with the volume control on the guitar, but as mentioned above, using thsi control will most likely affect your tone. Also a volume pedal is a lot simpler to use. Just place your foot on the pedal and adjust the volume. This is a great pedal for doing volume swells or for quick fade outs of notes. Just make sure that you use a unit that doesn't affect your tone. Here's a few options: Dunlop DVP-1, Morley PVO+ or Boss FV-500L.
Noise gate/Noise suppressor
These are two pedals that easily gets mixed up. The difference is that a noise suppressor cleans up your tone and removes unwanted hiss and hum while a noise gate cuts off your tone when your signal is below a threshold point. The noise suppressor removes unwanted sounds while a noise gate closes so there won't be any sound between two separate notes - a nice tool when you want your sound really tight. Some noise gate options includes MXR M153 and Pigtronix Gatekeeper Noise Gate while some nice noise suppressors out there is the MXR M195 and the Boss NS-2.
An equalizer is a unit which can set the parameters for the frequencies coming out of your guitar. Sometimes they are used as boost units but often they are also used for tweaking your tone with fine tuning of bass, mid and treble frequencies. With the equalizer you can shape your sound and add the last little piece that is needed to cut through the mix. It also often works as a little preamp, which can come in handy. There's quite a few pedal equalizers on the market, to name some of them there's the Boss GE-7, the Empress ParaEQ and the Electro Harmonix TubeEQ.
A loop pedal records what you play and then loops it over and over again. This is a great practice tool, simply record a vamp of lets' say four our eight bars, loop it and play a solo on top of it. But sometimes it can be a cool and useful live effect as well, especially is you're playing a song that is based on a riff or some chords that follows through the whole way. For an example Steve Vai did this on the G3 DVD "Live in Denver" on his opening song "I Know You're Here". Singer/songwriter KT Tunstall uses it all the time in live situations and I've seen Beck using it as well. Some neat choices are the TC Electronic Ditto X2 Looper, the Boss RC-30 and the DigiTech JamMan.
A multi effect pedal is a whole lot of effects crammed into one single unit. These pedals are programmable so when you've created a sound that you like you can save it to a preset. These units are of course very friendly to your wallet, but there's some downsides too. They are often limited so that you can't use too many effects at the same time, which means that you will often have to choose between flanger, phaser or chorus and cannot use them at the same time. To turn on and off effects live can also be a problem because of the presets - if you've got a song that has chorus for one part, flanger for another and delay for a third part it can be hard to execute this on a multi effect pedal. And of course the sound quality isn't as good as when it comes to separate effect pedals. Sure, multi effect pedals these days has come a long way, and some of the effects in every unit sounds great, but rarely all of them. Also, these are digital units so forget about any analog effects. But if you're new to effects and want to try many for a reasonable price, then a multi effect pedal is a great choice for you. And maybe you find a preset that really rocks - it might be better than anything you've heard before - then go for it! Use it! If you don't switch around a lot and use the same sound for many songs, then maybe you'll find just what you need in one of these units. Some nice options are Boss GT-100, the DigiTech iPB-10 (which requires an iPad), the Line 6 POD HD400 (or the brand new Helix) and the Korg AX1500G.
Of course there are more funny pedals out there, some of them are really one of a kind and makes noises you wouldn't dream of. For instance, go check out the Death By Audio Robot pedal, it's one of those crazy pedals that are very hard to point out which category it belongs to. And it really doesn't matter, the importance when it comes to pedals is if you like their sound or not.