I too am in this position, often flying on Embraer and Canadair regional jets. With connection times getting shorter, and the wait for plane-side checked bags getting longer, I needed a bag that would fit in the overhead bin of these small aircraft.

I bought a Brookstone Dash Rolling Office, $99. It fits nicely (horizontally) in the overhead bin of E145, and also CRJ200, CRJ700 and CRJ900. Fits in Explus equipped planes. Haven't had opportunity to try it on CRJ100. Does fit under Dash 8 seat as well.

Its actual maximum outer dimensions (including wheels and handle) are:
8 3/8" thick
15" tall - standing on wheels
18" across