Ohhhh, yer making me type...
In the key of C frinstance, your diatonic triads (chords you can make with three fingers) are:
C, Dm, Em, F, G, Am, and Bdim
As you use more fingers you get the diatonic chords plus their extensions. You can substitute these for the triads for different flavors. Examples:
I: C, CM7, C6, C9, C6/9, CM9, CM13
ii: Dm, Dm7, Dm7/11, Dm9, Dm11
iii: Em, Em7, Em7/11
IV: F, FM7, F6, F9, F6/9, FM9, FM13, F9+11, F6/9+11, FM7+11, FM9+11
V: G, G7, G7/6, G9, G13, Gsus, G7sus, G9sus, G13sus, G6, G7/11
vi: Am, Am7, Am7/11, Am9, Am11,
vii: Bdim, Bm7b5, Bm7b5/11, Bm7/11 with no 5th
Whew!
So, say you had a I - iii - IV - V - I progression.
Thats C - Em - F - G - C
You could instead go:
C - Em7 - F9 - G7sus - CM7 or
C - Em7/11 - F9 - G9 - CM7 or
C6/9-C7 - Em7-Em7b9 - FM7-F6/9 - G11-G7/6 - C6/9
Those old school jazz guys were good, eh. They not only used these chords, but used different inversions of them, sometimes spreading them out across the keys for a light, airy feeling; sometimes playing them it dense little note clusters; using internal voice leading so there would be melodic content within the chord structure; and sometimes leaving notes out to only hint at the chords.
If you're really wanting to get into this stuff, you probably ought to get a book on the subject, even if you're only going to punch your chords into a sequencer.