I'm not even going to try to get into all the different kinds of jazz, but if you like 'swing', both old and new, then there's a whole community of swing DJs out there who have recommended listening and CD reviews online. Here are some well-known DJ's sites:

Jesse Miner
Ron Bloom
Reuben Brown
Paul Overton & Sharon Ashe

All of these DJs cater primarily to the Lindy Hop community, which mostly means the music they recommend is "danceable". That means you're not going to get recommendations on any bebop or free jazz, but plenty of recommendations on both older big band and more recent small combo / groove jazz.

Also, as a side note, almost all of this music comes under the Sony Bono Copyright Extension. It's not going to be public domain for a long, long time in the U.S., but it's already public domain in Europe. I have some cheap compilations I bought while I was in Germany (including a Polish compilation of Django Reinhardt). Probably your best bet for sheer volume of good stuff is, believe it or not, eMusic.com. They've got tons of old jazz music, and you can surf around and find the artists you like without blowing a ton of money.