Chess Opening Software Buyer's Guide
Club level players love to study openings, memorizing gobs of opening variations. But that's generally time wasted, because your opponents typically don't know the "book" moves and will deviate early in the game. So it's best for players to know general opening theory (principles which apply to all openings), the basic ideas behind the handful of openings they play (in other words, the general objective of the openings they play), and maybe a very few "book" lines to help them steer the course. Larry Evans (in the previously-mentioned Chess Life column) recommended that no player rated below 2000 USCF needs to spend significant time studying specific opening variations and that it's much more important to understand the ideas behind the openings you play.
Of course, most chess training material deals with the opening. It's what most players don't need but buy anyway and they're the easiest materials for authors to write. This doesn't mean that opening study of any kind is wasted; it just means that you need to be a bit more judicious with your selection of purchases regarding the chess opening.
I'll make a series of recommendations as a course of study for beginning to intermediate players. Then we'll layer on some additional material.
Chess Openings For Beginners
You should start with ABC of Openings. The premise of both versions is the same: that all openings share common basic principles. ABC of Chess Openings will teach you these general principles and then show you how they apply to specific chess openings. The latter part is not an encyclopedic treatment of all known openings but instead deals with some commonly played opening systems, illustrating how the principles apply to them.
Chess Openings For Intermediate Players
Once you've reached the Intermediate level it's time to consider learning the principles of a specific chess opening a bit more in-depth. There's a wide variety of opening training CDs available, but I'll recommend three of them. ABC of the Ruy Lopez and ABC of the King's Indian are highly recommended for two reasons: these are opening systems more than just openings (each covers a lot of territory with a myriad variety of related sub classifications), plus the DVDs contain a lot of instructional and entertaining video lessons. The third choice is Giuoco Piano, the old tried-and-true opening choice for beginners and intermediate players, recommended as a "first opening" by chess authors for over a century. The Giuoco Piano disk is a bit tougher than my other two recommendations because there's a lot more reading required, but it's still a good choice for players willing to do a little extra work.
I'll also recommend two "extras". Fritz Powerbook is a huge opening book which you can load into any of the ChessBase playing programs to give your chess engine access to a much greater amount of opening knowledge than is contained within the opening book which comes with the program. By using Powerbook you'll see a much greater range of opening choices adopted by your computerized sparring partner. You can also use the Powerbook as a reference, seeing the statistical success rate for literally every move in that vast game tree.
For intermediate players (as well as advanced) who want a veritable avalanche of opening theory at their disposal, I recommend the ChessBase Opening Encyclopedia. Literally every chess opening is covered by both theoretical analysis and complete games, making it the most complete opening reference available in electronic form. But be forewarned: the disk doesn't explain ideas behind the individual openings in text format and it provides its theoretical analysis primarily through symbolic notation (using the same symbols as the famous Chess Informant publications). While you can learn the theoretical ideas from just the symbols, you'll need to go into it armed with a good grasp of basic general opening theory (see ABCs of Openings above) and do a little "skull sweat" to figure it out.
For a look at the opening training disks offered by ChessCentral, have a look at this Chess Opening Software page.