Generally I'll swap equal for equal, I don't like window seats, but it all depends how the person acts when they ask me, as others have said if they're already settled in to my seat I'm less inclined to be a nice guy, obviously middle is out of the question (unless I'm already in a middle seat, which rarely if ever happens). I was offered a seat up front on an Airtran flight out of Sarasota when I was visiting my parents recently (I rarely fly AT and therefore have no status with them). The man sitting middle with me in the exit row said his wife was in first, and he'd be willing to swap my seat for hers, obviously I wasn't going to object. I assume in that scenario they purchased a first class upgrade the day of the flight and planned to use that as leverage to get seats next to each other.

I've only taken an inferior seat once, I was flying on DL from DTW - PHL up front. All of first was full and I saw a uniformed military man walk past to sit in the way back. After everyone was seated I realized there was no way the captain was going to bump him to first since it was full (in the past I've seen captains do this on DL pretty regularly). I pulled the FA aside and asked if it would be OK if I gave him my seat, she agreed, I gathered my things and got an isle seat in the very last row. I was happy to do it, I have an immense amount of respect for our service men and women. He thanked me, and I thanked him for his service, it was a true feel good moment I suppose.