I'll second the recommendation for Son Volt. 'Methamphetamine' and 'Windfall' are two of my favorites.
Try The Jayhawks as well. 'Blue' is catchy, same as 'Save It For A Rainy Day'
I've had some friends turn me on to some newer stuff lately, like Langhorne Slim and AA Bondy. I don't know if they're more folk than alt country. I'm over 30 and don't really care for what it's labelled as anymore, as long as I like it.
Others, (don't know if the qualify as alt) I like that haven't been mentioned yet :
Paul Burch & The WPA Ballclub
Jimmie Dale Gilmore
Wayne "The Train" Hancock
Billy Joe Shaver
Rank & File
Jason & the Nashville Scorchers
I like Uncle Tupelo, have heard a bit of and liked Son Volt and Calexico. Also listen to Wilco and Band of Horses regularly. Haven't really had a chance to listen to any of the other bands mentioned in this thread so far. This kind of music is very hard to come by, if one doesn't want to pay for it..