One of my favourites is,"Freeamp/2",
This is a quick jam track I did with AbletonLE demo and FreeAmp2 that I got with my sound card before finding MixCraft which I've just purchased which I prefer as I like it's layout and cost.
I stuck it in Mixcraft and applied a Master limiter and Expander just for kicks
I used a free jam backing track from this guy,
Great site for a quick practice IMHO and he doesn't mind if you download them and put your own thing to them so long as it's not for commercial use