I have an Akai Headrush II - massive fun & idiot proof.
It is volatile memory so will not store your riffs once you switch it off - that's why I bought it, to use live. Plus you can over dub real easy. Also has a cool sounding delay & tap tempo.
All in all its great live for looping on the fly & great for at home when you want to work out a two part guitar peice or just want to practice some fret wankery over 12 bar blues.