The Univ of Arizona Biology Project web site is fantastic. Great interactive tutorials on a bunch of MCAT-relevant topics (many of which are covered in intro-level bio classes).
http://www.biology.arizona.edu/DEFAULT.HTML
I also really recommend the CD-ROM that comes with the Campbell and Reece Biology textbook. It has great interactive activities, for those times when you can’t stand to read any more.
My experience with prep course online materials (I did all the online materials for one company’s MCAT course) was that they were not very good: they were just digital versions of textbook text, with no animations or other helpful learning tools. Definitely try before you buy - they are very expensive. Also, the online materials did not cover all topics, and some of them were impractically slow unless you had a high-speed Internet connection.