Chesstempo x Chessmentor for beginners

I recommend getting a diamond membership here and doing all the chess mentor courses. It will take you at least a year probably, but you will get a lot better. Use the tactics trainer 15 minutes per day for a year also. It really helps. 

I second what the "lawdog" said, but instead of trying to do all the courses, concentrate on the beginning tactics courses.  Don't just do them once and move on; do the elementary tactics courses over and over again until you have "burned" the tactical patterns into your brain.  When you can almost instantly identify the answers to all those lessons on forks and pins and skewers etc. then move over to the tactics trainer and test yourself.  When you're feeling a little burned out by studying, then kick back and watch a few videos.  I recommend IM Danny Rensch's live sessions as a perfect blend of rapid paced education and entertainment.  You will improve your chess.

I agree with the "get a membership and do Chess Mentor" crowd.

I would stress that you need to not do teh chess mentor courses in the "default" mode of randomly selecting lessons based on performance. Rather, pick a course, do it sequentially, and then pick another course and so forth. 

Doing the course in rating order is ok for the most part, but pay attention to the course titles as sometimes a couple of courses go together and should be done together even if the "rating" is a further apart. 

The bad dog is right, do the simplest checkmate and tactical courses first and learn them well. I couldn't even do the easiest tactics trainer problems until I learned what the tactics were from chess mentor. It's well worth the effort, it really opens up the world of chess for you. 

I have both premium membership here in chess.com and chesstempo, I find that the variety of problems complement each other, and I alternate one day here and the other there, to keep in fresh; chesstempo membership is not extensive at all, and the game explorer is cool also there.