Karma is a philosophical concept probably summed up as a sum total of everything that an individual is doing or has done. Wikipedia does a better job of explaining it. A combination of collaborative categorization and filtering (folksonomy) with peer ranking (as in Ebay) would make for a good karma bank.
Take for example, the problem of good travel advice and services and finding such on the internet. One could come up with a system where a would be backpacker would pose a question asking for a place to stay at
a place he/she is travelling to and possibly also a travel guide when he/she gets there in exchange for something other than money, possibly a similar arrangement for future travels to his/her own hometown.
Another person responds saying he/she would like to take it up as an offer. This person has a karma rank listing all the good he’s done in the past. The tag for this would be travel or perhaps backpacking.
The concepts describe here borrow from systems developed by various organizations including craigslist.org, various auction sites, del.icio.us etc. It’s not meant to be a barter system though. It matches the economy more along the lines of burningman.
There are various hurdles to this of course, and none of them technical. There is a problem of bootstrapping, getting enough people with enough karma online to get the system chugging along and for newer people to trust the site.
The other more fundamental problem is the problem of good intent, not everyone will have it. There is bound to be malicious people trying to take advantage of the system for rapid gains. But then again, there are sites out there already which have defied this and are still going strong.