Don't do it Nathanael. If the painter, plumber etc works out you won't get a thank you and if he doesn't you get aggro because YOU recommended him.
Also because of what we do we are regular visitors to the same houses and for some reason it's automoatically assumed by our customers that we're mates with painters, plumbers, chippies, electricians etc etc.
It'll be a one-way street in that you'll be dishing out other tradesmen's cards (because you've been asked the question) whereas I'll guarantee nobody will ask your plumber "mate": Do you know of a good window cleaner?