John,
If you are looking for classical, you will definitely want to check out this software package.
www.smartmusic.com. The program is a free download, so you can check it out at no risk at all. It's huge... around 500 meg. It was originally developed for band instruments, but they recently added string support. One of the features is 'music on screen'. They have the Suzuki strings books on there for Bass, Cello, etc. Lot's of classical Mozart, Bach etc. Music is on screen with nice accompaniment that will actually follow YOU if you are plugged in with a mic. It is truly amazing software.
My favorite is the jazz section. Lots of the old standards by Duke Ellington, Aebersold, Wynton Marsalis etc. and you can select from bass transcription, lead sheet view, drums, piano, you name it. The recordings are not MIDI, they are very high quality sounding instruments that you can adjust the tempo to your ability. You can select or de-select various instruments.
To unlock the entire library involves a subscription. If you are a student, you can get it for like $20 a year. I've used this software for a year now and it has hugely improved my playing ability.
Hope this helps!
Jeff Fletcher
