#1
Situation: A drummer sits down and starts banging out a beat. You want to add to the music, so you pick up your guitar and you play ______.

are there any experienced rhythm guitarists out there that can tell me some nice/funky/bluesy/rock sounding chord progressions, or contribute some wisdom about what they might play in a situation like that? thanks.
#2
A-C-G-D?

Really all it is is taking a prgression that's pretty generic (^that one e.g) and adding your own spin to it, for example I like to change from the major chord to the suspended chord and back again quite a lot. Other people like to play the chord, then do a little scale lick in the pentatonic of the key (zOMG: USUALLY means the chord the piece started on). The style of play really depends on your influences and YOUR OWN style of play (duh). To help you along, here's a list of pretty generic chord progressions that you can use and put your own "spin" on:

A-C-G-D
E-G-A-B
D-A-C-G
C#-B-E-F#
A-G-D-A
A-C-D-E
A-G-F#-F OR A-G-F-E
E-E-A-E

And ofc the magical 12 bar blues one: A(4bars)-D(2bars)-A(2bars)-E(1bar)-D(1bar)-A(2Bars)
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#3
Good list^^.

Basically, just learn the basic or "cowboy" chords and learn how to put them in effect.. As far as I know they are: E, A, G, D, C. And they're all in the first position. Once each chord is mastered and you an easily transition, just tinker with them and have fun.. That was the first "Rhythm Exercise" I ever learned. It can be anything you want it can have just 2 chords, or all of them. Take your pick. :p