Which sites offer good tutorials for novices to improve? Can anyone recommend sites that they have personally had experience with to be good for learning by novices? I play chess regularly, but I have never studied theory and while I am doing fairly well against most of the regulars I play against, there are several people who are able to have a clear advantage by the midgame.

I'd like to improve my openings by studying theory, but I've never looked at it before, so I wouldn't be helped much by a dry treatise on how to develop a pawn line against, for example, 1.d4. I'm looking for an objective analysis of websites that teach the theoretical principles of openings, with good examples and a minimum of references to various famous openings.
IMHO if you are struggling with the middle-game it makes more sense to study that particularly (as well as the endgame) instead of focusing on openings. I would advise you to learn to record your games in Algebraic Notation in order to learn from your mistakes (preferably with a stronger player). As far as online material you can use I can recommend Novice Nook by Dan Heisman and Modern Ideas in Chess by Richard Rèti