I one loaned an unreliable yet close friend $300 to buy a Palm Pilot that he said he needed for the business he was starting to keep his schedule. I felt like a loan shark, having to hunt him down on a regular basis and threaten him to pay up. It took him 24 months to pay me back.
I also paid an ex-girlfriend's utility bill for 6 months. It took her more than a year to pay up, but she was broke so I'm lucky I got any of that back.
When I was in college, an entire sorority mooched off of me at least once a week on bar tabs. (I worked full time and had a ridiculous amount of disposible income in college.) They never paid me back, but I wouldn't say they got off free either
I'm sure some still take advantage of my good nature. But I don't mind someone not reimbursing me $5 for a couple pieces of my pizza they ate. I've learned to stay away from the bad moochers and don't keep track of the little stuff, or I'd be pointing the finger at everyone.