The Princeton Review
Details: $100 Off Classroom and LiveOnline Courses + Tutoring for SAT, ACT, MCAT, LSAT, GMAT, GRE, DAT, OAT.
eprep.com
Details: 15% Off of everything (I think!). It gave me a 15% discount on the ACT test prep
Prep Me
Details: Save 10% on all purchases!