It would be great to compile one!
For the Pacific Northwest:
Washington (primarily Seattle area): nwhikers.net; wta.org (the second is not a forum but compiles trip reports and has a good guide for finding hikes)
Oregon (primarily Portland area, including SW WA): portlandhikers.org, which in addition to its forum has an excellent field guide, similar to that of wta.org.
One of the best uses of local forums, IMHO, is to find out about current conditions. With our Northwest winter refusing to go away (still snowing down to 2500' and our snowpack now close to normal after being way below normal until mid-March), a lot of folks planning to hike in June around here are going to be very disappointed unless it suddenly turns off really hot!
Googling can often help. That's how I found nwhikers.net, which led me to portlandhikers.org a couple of weeks after the latter started up.