http://fallout.wikia.com/wiki/Karma_(Fallout_3) gives the karma points for centain actions.

i like having clover for a sidekick but some locations are too dangerous for her. in those places, i would like having fawkes along. but clover needs an evil karma and fawkes needs a good karma to join.

giving psycho to paulie cantell results in a negative karma of 100 per psycho. this is renewable. you can only give him one at a time. but you can start talking to him again right after he takes the psycho.

so using the link, you can use renewable ways to quickly change your karma.

Actually, Paulie Cantell isn't renewable. Give him some Psycho, and he OD's and dies. If you're quick, you can squeeze more negative karma before he finally dies, but I wouldn't call it "renewable." Giving water to the beggars and donating to the Church are renewable (only good karma,) as are trading in ears/fingers (for the Contract Killer/Lawbringer perks, Evil and Good respectively.)

But what's the point of this thread? It's practically common knowledge by now on how you can increase/decrease Karma. And there's nothing on that link...