Rare Funk Liberation, Vol. I by the UK label Goldmine/Soul Supply contains two things in spades that make it a fantastic listen: 1) 18 tracks of rare, deep, hard funk from a bygone era (late 1960s & early 1970s). 2) A relatively large percentage of female vocalists. While the CD is out of print, you can find downloads of it various places online.
Track 1, "Good Things" by Pearl Dowell:
Track 6, "Strike" by Union: