For realistic characters, weaknesses and flaws are often more important then strengths and coolness. The links below are good for creating more rounded characters.
Creating Original Characters... This appears to be more for screenplays, but I found the information interesting.
Creating Character Flaws: I skimmed this. It looks good. Information written for guys who are gamers. But the information does look promising. (Can a character be too awesome?) :P
Character Flaws: This is definitely about RPGs. But if you scroll down, it has a list of flaws.
Strong Characters versus Weak Characters: The author takes the challenge to figure out exactly what a "weak" character is.
Creating unforgettable characters: This is a little preachy (not religious,) but the information looks promising.
The Spice and Wampum of Characters: This piece is interesting because it talks about the basis of a character's flaws. What caused the flaws. (Please note the examples are generic. Not all little boys raised by misogynist fathers become wife beaters. Some do.)