Best Chess Instruction software?

What software/games are the best for improving chess skills? I don't mean building up from scratch, but I mean for improving skills from an intermediate level? Fritz 11? Chessmaster? What other non-game or game-instructional software is good?

Well, since you asked, I have to mention:

Chess Mentor!

Convekta publishes Chess Tactics for Beginners, Chess Tactics for Intermediate Players, and CT-ART (advanced)--these are fairly traditional options. Personal Chess Trainer is newer on the scene but also highly recommended--it includes tactics, strategy, and annotated endgame modules. These all assume you already know what a fork, pin, skewer, and discovery are--if not, you should begin with something that also includes beginner lessons like Chessmaster: Art of Learning (it also has intermediate lessons).

Since the time I made this thread, I've gotten Personal Chess Trainer, Chessmaster: The Art of Learning, and Fritz 11. Unfortunately I haven't been able to find any of the Convekta programs, but I think I have enough for the time being.

Good stuff! Convekta's software is available at store.convekta.com. But if you own and are happy with Personal Chess Trainer, you won't need Convekta's software. They are more or less competing products in the tactical training arena. :)

Lucas Chess is free and has an incredible amount of training features, and hundreds of engines to play against all all manner of rating levels.