Yeah, I'll second Agent. I also use it for all my personal email. I'm still using my version from 1997 and it still kicks ass.
What makes it so useful for usenet is that its keyboard commands, and its system of ignoring certain threads and all crossposts with a single keystroke, lets you rip through a regularly-read newsgroup in record time. There's a bit of a learning curve to overcome, but once you're there, it's too cool.
At least, that was back in the day when I read Usenet regularly. I don't any more...