No real brainwaves around sites to listen to Jazz music, though e-music is pretty good, and allmusic, if you want to do a little sampling, is at least very comprehensive.

Not to discourage you from doing your own homework and thinking on this, but there are a plethora of "Top 50/100/etc" jazz listings on the web (of course), and I though it might fuel discussion to throw a couple of them out there. Ours will be better. These each have different "rules", which I think we could clarify as well (per my last note).

Site - I like this one because it has a real free jazz leaning - lots of music to check out.
Site - this one and the next are more straightforward in their choices.
Site