The Exchange maybe? It's a network of anonymous people who do favors that range from "put a flashlight here" to "leave this door propped open between 1200 and 1215" for "karma" which means favors like that will be done for you or you'll get rewards no money though.
Unwired pg 83, Seattle 2072 pg 191, Runner's Companion pg 130
It's gone through various versions, but my old books are packed away and I'll be running off memory, one added an anonymous chat function.
You could also be thinking of shadow want ads, basically like classified ads from today. I remember something like that too, however I just think it was part of Shadowland or the Nexus that was housed in that monument.