One of those is feedback roulette. You have to review other sites and at the same time others will review yours. This is as easy as it gets. Help me and I'll help you. It's a true win-win.
There seems to be many more services which talks about favors, but those are more about making "a favor" for money or bonus points. So I don't think that those are favors as such, but those are just plain work.
Then there's services like Pay with a Tweet, where you can get your goods , by making a favor of tweeting about the thing you received.
Also there are at least couple of more generic tries where people can exchange favors, like Favorpals and Favorfly but neither of those doesn't really seem that successful yet.
Still I believe there is room for this in web. For general service offering favors for real life happenings, web might be too big. But for niche services like getting feedback of your peers in your area of expertise, this is a perfect way to increase co-operation between people and it will create lot of good thinks to the world, when smart people help each other to get things done.
Written by +Henri Hämäläinen