This is just a random thought that occurred on the train home. A friend was talking about how it might be tricky to do SEO for a plumber or electrician. I didn't quite agree, but it occurred to me that for a local business marketing such services in the interweb age, there's some quite clever things you could do.
First, join Twitter. Make sure you mention you're a plumber and your location. Talk about your work and be generally personable; Being genuine is always a good start on the web. Include your website link so people can check your plumbing credentials, it's probably best to mention you're in Yellow Pages too, not everybody believes what they read on the web...
Second, go to twitter search and search for "burst pipe" or better "need plumber" or any of the other hundred complaints you might help with. Check it has lots of results. Go to advanced search, repeat, but with a geographic limit of 50 miles of your location. Say you're in London (Oh look somebody needs help).
Go to Yahoo Pipes. Create a pipe with all those queries merged together. You could limit them to the last 24 hours if you like, so you don't end up trying out of date info. Subscribe to the feed. Watch the feed for appropriate tweets. Respond when you see them by an @ reply or by following the tweeter and introducing yourself. Introducing yourself is important, hopefully shows you're not a bot. You might want to give a special rate for twitter referrals. Hopefully your serendipitous contact will be seen as a boon, especially if you don't behave in an autofollowy kind of way, get the job.
Do a damn good job. This part is important.
Hopefully your client will talk about the novel experience of being contacted via Twitter at just the right time. It'll be remarkable, after all. This might net you some interest from friends of friends who need work, or some press attention, all of which would be good.
Repeat until you have a network of satisfied customers who will always remember you as the twitter plumber.