For owner-built home advice, the CountryPlans Design/Build Forum.
The best help I’ve found for my area of interest, digital recording and the restoration of vinyl records, is the now-defunct, but newly archived Syntrillium Forums. I used Cool Edit 96, then Cool Edit 2000, then Cool Edit Pro. This program is so technologically advanced, it will do things that I think the designers never even considered. It took a whole community of users to come up with new uses for the functions, and to be able to explain what they do and why.
I spent months and months just reading the articles on noise reduction and click/pop removal. Then I discovered that a guy came up with a foolproof method of doing the hard work of vinyl restoration in one, automated process. It made all of my previous results seem trivial by comparison. Now, on my restorations, you can’t tell that the music you are hearing used to be on a record.
After Syntrillium was sold to Adobe a couple of years ago, the forums disappeared. But a search reveals that they have been re-hosted by Audiomasters.org.
An astonishing number of forums all about gardening
http://forums.gardenweb.com/forums/
Finding good deals on Mac related technology
http://dealmac.com/view.html
The best and biggest digital photography forum
http://www.dpreview.com/forums/
The best and biggest forum on “VCD, SVCD and DVD Capturing, Encoding, Authoring and Playing” (I usuallu use the Mac Video sub forum)
http://www.videohelp.com/forum/index.php
Good forum for digital design on the Mac…great collection of tutorials
http://www.baseboard.net/forums/
For repairing and maintaining old tractors.
Not that my half-century old Ford doesn’t run like a top… cough
Bad Astronomy for your cosmic needs.
For legal advice, FreeAdvice forums can be helpful.
RailPage Australia for my railway fix. I spend more time there than here these days.
The Bob Dunsire forums for all things piping related. Not restricted to the Great Highland Bagpipe, although that is the main focus - also includes drumming, dancing, and other types of bagpipes, such as the Scottish Small Pipes, the Irish uillean pipes, Northumbrian pipes, and so on. Also includes some discussions of celtic languages, history, etc