Nancy Drew Games!
Her Interactive, over a period of several years, has come out with a series of Nancy Drew computer games (and one Hardy Boys game). There is currently a total of 23 games, and more will probably be added. Some of the games (The Secret of the Old Clock, The Secret of Shadow Ranch) are based on the classic books that many of us grew up reading. The oldest ones appear to be based off of the "Nancy Drew Files" series, and the newest ones were made up by the game designers.
I've played nine of these. The oldest ones weren't much fun. Even with walkthroughs, my brother and I could never get very far in the game. The first ones we were able to finish were Treasure in the Royal Tower (#4) and Ghost Dogs of Moon Lake (#7), but for most of those we depended on outside help from walkthroughs. By The Secret of Shadow Ranch (#10), though, the overall design of the game had improved greatly from the initial games. The Curse of Blackmoor Manor (#11) is personally my favorite (werewolves, ghosts, and carnivorous plants, oh my!), and after that is when the games finally get really, really good. They're hard to beat, but my brother and I were able to solve Last Train to Blue Moon Canyon (#13) and The White Wolf of Icicle Creek (#16) mostly on our own. These are the newest ones that I've played, though, because I prefer buying books to spending $20 on a computer game.
Her Interactive charges $20 for each game (if you go to other websites, however, the price may be significantly lower). Their website also has free mini-games, activities, and demos of several of the twenty-three Nancy Drew games.