Sites for practice GRE
Here are the practice GRE subject tests. They have introductory material, the test, AND answer keys with them. Note though, they are .pdf files, and about 60 pages long,
so only print if you have the time to wait AND the paper in your printer!
Biology Test: http://ftp.ets.org/pub/gre/Biology.pdf
Biochemistry, Cell, and Molecular Biology Test: