From the College Board
All previous years’ free response questions (with solutions)
Online self-grading multiple choice (from Georgia Tech)
Project Euler (small mostly mathematically-oriented challenges)
Really good suggestions and background on the test format here. In particular, note this passage:
Statistical analysis of published results from the 2004 exam shows that over 98% of students who got at least 27 out of 40 on the multiple-choice section received a 4 or a 5 for the whole exam.