Added to in March 2010.
Well, it was the same story yesterday as it is every year. I decide not to do my bit for the RSPB Big Garden Birdwatch in the morning, but later in the day. Later in the day, of course, all the birds have vanished. Yesterday, just one blackbird, one wren and two sparrows were present, even though earlier there had been more sparrows, and, of course, outside the front of the house, where I was not counting, there were more too.
I am sure there are good explanations for this, although there is not really a good explanation for my continued refusal to do the survey in the morning!
This birdwatch is an example of "citizen science", science data gathering using as many people in the general population as want to. This enables interested people to participate in scientific research, and might, I guess, also increase those people's knowledge.
The RSPB are interested in seeing how many of particular birds appear in your garden (or park) at one time. You can only count your garden, and you can't count things that just fly over. When you report the results, you can give information about your garden - size, distance from fields, types of trees, that sort of thing. Because you can't count other people's gardens, we have to ignore the trees that we can see from our house, and this means ignoring the crows that are nesting nearby, but the hope is that the owner of those trees is also counting.
Some time ago BBC Radio 4 ran a series on Citizen Science, and the programmes are still there to be listened to. The series covers other initiatives - SETI@Home uses home computers to analyse radio frequency data from space for patterns, and other initiatives do similar things to molecular structures looking for ones that might make good medicines. These use computers' "idle time".
Wikipedia also has an entry (that needs work) that lists some initiatives, primarily American ones.
Addition: Have just (29 March) come across this, in Research Information. It discusses a JISC project, as well as giving more examples of citizen science.