Oh so many, but the only one that really springs to mind is 'The Red and the Black' by daphnaea. It was so beautifully written and the characterisation was so believable (even if the plot wasn't) and complex that I'd be lying if I said this fic didn't influence my picture of the boys, and their relationship with each other.
Aside from that, 'The Scar' by mysid built a very nice Remus for me to angst over and now that I've thought about it, influenced my recent piece over at the R/S Games.

I mean, I can give a loooong list of good fics I've read and would recommend, but that would be digressing from the question.

To add to the SBP discussion nerak_rose, I really enjoyed it as a fic. I enjoyed the writing style and got very involved with following the story. And though it will no doubt have influenced some part of how I see the boys (without recalling exactly what), I don't consider it a bible or canon either.
Also, was it you that mentioned 'stealing Harry'? I've never heard of it before...