What is a pitch shifter?
A pitch shifter pedal takes your signal and modifies the pitch of the signal in some way. There are many options - down or up in different intervals. A classic interval when it comes to pitch shifters is an octave, either up or down, that can be blended with your original signal or sound on its own. There are also pedals on the market which can harmonize your tone with many different intervals.
Any good examples?
One of the earliest pitch shifting pedals was Roger Mayer's Octavia, a pedal made famous by Jimi Hendrix on the song "Purple Haze". This device added a signal one octave up from the original source and added some fuzz as well. After that pitch shifting devices fell out a bit of fashion until some players discovered the Digitech Whammy Pedal in the early nineties. Trent Reznor of Nine Inch Nails was an early user of this effect but the one who really comes to mind is Tom Morello of Rage Against The Machine. When he pitched up his tone two octaves on the "Killing In The Name" guitar solo it was something completely new. After hearing that one guitarist after another ran to the music shops to get their own Whammy pedal and both Joe Satriani ("Cool #9") and Dimebag Darrell of Pantera ("Far Beyond Driven") used it. However, Tom Morello showed some years later once again who's the king of the Whammy Pedals on Audioslave song "Like A Stone", combining it with tasteful delay. Alternative pop acts like Radiohead ("Just") and Muse ("Invincible") has also used this pedal to create octave up effects while Jack White of The White Stripes used it on the octave down setting to create a bass-like sound on "Seven Nation Army" (no, that's not a bass guitar playing that riff, something that should be pretty easy to figure out when you know that this band only used guitar and drums).
Which pedal should I get?
As you can see the Digitech Whammy pedal (these days there are numerous different models, so make sure you get the one that suits you best) is a really easy answer. But it all depends on what you want to do with your pitch shifter. If you're looking for that Hendrix sound you should look for the Roger Mayer Octavia, or another similar option like the Rocktron Purple Haze. If you're more into harmonizing single notes into sounding like two guitarists (great if you're the only gutiarist in a band that plays a song by a band like Thin Lizzy) or more (something for the Brian May wannabe) maybe you should check out the Eventide PitchFactor or the Digitech HarmonyMan. A simpler, but more limited, choice is the Boss PS-6. There's loads of pitch shifters out there and the one you should get depends a lot on your style of playing. Check some of them out before you make up your mind.