favourite chord progression in a jamsession

Yeah, the twelve-bar blues can get a little tedious, but at least it's something everybody knows and is comfortable with.  

Ew blues. 

I usually like major scale progressions, skip some notes to make it pentatonic sometimes.

I like | Dm | Dm F G |, and within that, i like to play around with chords like Dm7, Am/D and G/D.
Sounds really nice when played quite slowly on Electric Piano with some Phaser & Vibrato effects!

And then there's the one from "Dogs" by Pink floyd:
:| Dm Am/D | Dm Am/D | Dm Am/D | Dm Am/D |
Bb | Bb| Aaug (Also called A+ or something) | Aaug F Em|:

I think that's correct ^

And the one from "Starship Trooper: Life Seeker" by Yes:
:| G Eb | C |:

That you can play around with by using diffrent chord shapes, and chords like Eb/Bb. Very fun to play!

What I love to do is start with a basic chord (or progression) and then... twist it around a bit.  See what I can come up with in that chord to make it sound a bit different and then carry on the progression from that initial idea.  I hope that helps your jamming a bit!  Sometimes it can inspire epics ;)

I actually do change progressions while jamming mostly, though usually the theme is based on a few simple chords like Em (say, Autumn Leaves). It doesn't mean you have to keep the chords the same. I like to add progressions between two chords, such as my all time favourite Am7 - D/D7, which is used throughout the Pekka Streng song "Puutarhassa" which has become quite a cult song already here. Am7 and D gives you a lot of space for changing and improvising both with the lead and the chords. I've seen a lot of pianists run through the board some times. I fancy a chromatic descending from A to D, or just running the a minor scale like Am7, Gm7, an F- variant, E variant and a D7, a bit faster, what ever I feel like, or what ever the guy playing the solo is playing basically. Gmaj7 seems like an easy choise often too.

comping chords over a vamp is always cool, as well as free form, love it!

but chord progressions i like to do besides the standard blues progression are for example...

the lydian pattern of (for example) Eb major to F major and repeat

G minor 7th to c major 7th

D major to A minor 7 to G major to B minor 7 to C minor 7 to Bb major 7 back to A minor 7