In short, I am looking for an online free chess engine that has either: a.) multiple engines with multiple ratings, some as low as 1500-1600, and some as high as 2200 or better, or b.) is a single engine with adjustable difficulty ratings from about 1500-1600 and up.
To clarify, someone once recommended that when playing alone against the computer, I should find a difficulty setting that I can beat roughly 2/3 of the time, and another that I can beat only about 1/4 of the time. Of course I'd rather play OTB (the best practice) but life/job/family commitments don't allow for too much of that these days.
I can readily beat Microsoft's Chess Titans at all levels, including level 10 (or I should say I can beat it at least 3 out of 4 times). Thus, Chess Titans doesn't fit the bill of being able to beat me somewhat more often. I realize that most purchasable standard chess software has adjustable ratings, but I don't feel like buying any right now.
Lastly, I am aware that Elo ratings for chess engines are a fuzzy subject -- given that they don't make the same kinds of blunders that real humans do, don't give up, play at different levels in different situations, etc. I am not demanding a highly specific rating. I merely would like to vaguely know the general level of my computer opponent while I play.
At any rate, any recommendations for a free online chess engine with adjustable difficulty levels, whose elo ratings are roughly known?