Software for training openings?

Can anyone recommend some good software for learning openings? I'm thinking very basic stuff, similar to tactics training, except for openings. For example, I make a move, the computer makes a random book move, I make another move, and the computer beeps and says "try again" if I ever go out of book.

I know, I know, I'm supposed to "understand" openings rather than "memorize" openings, but I still think this would be a good exercise, to keep me sharp.

Here is an excellent free openings trainer for PC users, but you have to enter the moves you want to train into it. Other than that, Fritz 12 has an opening training feature.

I like Chess Openings Wizard, previously known as Bookup. You can either buy ebooks or enter the move yourself which is what I usually do. You can also import pgn files into the program.

I like it better than Chess Openings Trainer but they're pretty similar although COT is free.