Band in a Box is a great program that will play melodies and rhythm while you practice along. It's a snap to enter chords and tempos can be changed instantly. This is a great program and is highly recommended. Download "Fiddletune" style, for Band in a Box. To use this after you've downloaded it, click on the "Sty" box next to the song title box, and then choose "rebuild styles". This lets Band in a Box recognize the new style.
Every year the International Bluegrass Music Association (IBMA) meets. Great jamming. Here's a page from IBMA 2000