If you like Umphrey's McGee, you might want to check out
Internet Archive
If you aren't already aware of the site, they have a huge live music database of bands, including Umphrey's McGee, available for free listening/downloading. Some of the shows are soundboard recordings and some are audience member recordings.
Many bands listed there would probably fit into the jam band category.
There is a forum there devoted to the Grateful Dead, and many of the participants are quite knowledgeable about the jam band scene.
I visit the site often for music and info about bands/artists.