CyberBass tracks for each part of :-
These let you listen to the whole thing, or just your part with the others in the background.
What's more, you can try it at anything from 25% above the correct speed to half speed.
Choralia tracks for each part of :-
Much the same as above but with synthesized voices and you can go 50% faster.
ChordPerfect tracks for each part of :-
This one has real voices and instruments. 'Slow' can have some wobbly effects on long notes!
If you know and like any others, please let me know and I'll add them,