Behringer EQ700 EQ Pedal
This is a 7 band EQ pedal from Behringer which can take batteries or be plugged into a regular 9v supply (this is how I use it).
Only £20 and actually well worth it. I found it didn't suck any tone at all unlike many Behringer pedals. Yes, it's a copy of the Boss EQ but much cheaper. Just don't turn the level up too high else you'll introduce hiss - keep below +10. But then again the Boss EQ pedal will also add hiss at high db.
What is it for? Helping you get the perfect tone. Try on your amp first and then use the EQ as needed.
I found the actual EQ to work very effectively, with adjustable EQ at 100Hz, 200Hz, 400Hz, 800Hz, 1.6kHz, 3.2kHz and 6.4kHz. This will allow you to boost the low-end, maybe chop out some muddy sounds (often around the 200-800 area) and then add a bit of top end sparkle.
Boss shaped and size pedal, so sits neatly onto the pedalboard. It's yellow, although not a lurid yellow, so you won't miss it.
EDIT - June 2011 - I've been A-B-ing with this pedal in and out of my pedalboard and would conclude there is a very slight loss of tone to make the amp sound less 'full'. However, you can 'put back' what the pedal takes out, by using the EQ.