What is vibrato?
Vibrato is something that occurs when the pitch of a note is moved up and down at a regular speed, a cornerstone for musicians and often something pretty personal and recognizable. Vibrato can be achieved both with the fingers on the fretboard and with the whammy bar, but also with an effect pedal. The tone of your guitar is then processed inside the pedal which adds vibrato to the tones coming out of the unit.
Any good examples?
Vibrato pedals are pretty rare and not used very much, but there are some player's out there who likes to modulate their sound with this effect. Graham Coxon of Blur is one of them, and a result of his use of this effect can for example be heard on "Oily Water" and in the solo part of "Coffee & TV". Two other really cool guitarists, Nels Cline of Wilco and Omar Rodriguez-Lopez of Mars Volta, uses vibrato pedals occasionally which can be heard on songs like "Impossible Germany" and "Empty Vessels".
Which pedal should I get?
The Boss VB-2 is discontinued and pretty expensive on the second hand-market but it's a great pedal, used by all the three guitarists mentioned above. Behringer UV300 is a much cheaper option, but sounds less vintage. Other pedals on the market are the Electro-Harmonix Wiggler (which also has a tremolo setting), the SIB! Mr Vibromatic, the TC Electronic Shaker Vibrato and the Diamond Vibrato Pedal. If you want a vibrato pedal, think about how much you will use it and then set a budget for how much you're willing to pay for one. If you're really into this effect then spend your money on quality, otherwise it might be wise to save some money on a cheaper option.