Using the custom search engine I don't have to specify that I'm searching for Maxima, Octave, R or SciPy stuff, because I already told Google to favor results from those pages. Right now the custom search engine (CSE) only has about twenty sites included here's the big ones (list updated 10 Nov 2009):
And of course the mailing list archives for those projects as well. The idea of this CSE is to help the user find articles, tutorials and documentation about using open source math tools to do specific tasks.
The results are pretty good. I choose the option of including broader web search results, so high ranking results from Wikipedia or MathWorld still show up, but the first page is full of relevant links to the favoured sites. For instance the results for 'bayes model averaging' have a mix of about half and half papers from academic websites and links to various R project pages, or the results for the Maxima function augcoefmatrix() which links to the Maxima manual as well as several web sites with usage examples.
It really improves the signal to noise ratio. Leave a comment if there's a site you think I'm missing.