I ran across a good (and free) printable circle of fifths on www.tikiking.com
It prints out at roughly 3.5 inches (90mm), so it's a good size to toss in a case or gig bag.
Taken directly from the website:
The circle of fifths can be used for many things. To help with chord progressions, to determine the Key that a song is in, to determine the scale for that Key.
The "circle of fifths" is a term coined by German musician Johann David Heinichen in 1728, and is basically a chart showing all 12 pitches which are arranged in such a way that as you go clockwise, you go up a fifth. However, due to the nature of how scales work, when you go counter clockwise you go down a fourth. For example, the fifth note of the G major scale (G A B C D E F#) is D (going clockwise) but the G is the fourth note in the key of D (D E F# G A B C#)
So, if you pick a key, Say "C" for example, That is your I Chord. The adjacent chord counter-clockwise (left) is the IVchord. The chord clockwise (right) is the V chord. So, in the key of C major, C is the IChord, F is the IVchord, and G is the V chord.
It has been said that nearly half the western worlds songs are based on the I, IV ,V progression. So there you go...