I'm a member of two forums:
http://gardenworld.proboards.com and
http://www.gardenerscorner.co.uk/forum/ but I've never really got into them in the same way as YACF. Different demographic and familiarity with computers mostly.
I follow loads of blogs, but that very much depends on what you're after and whose style you like. For example, I follow two of James Alexander Sinclair's blogs, I find him mildly amusing but understand that he's not to everybody's taste.
Others are:
Emma Cooper - into growing unusual edibles
Jane Perrone - Guardian gardening correspondent, a bit quiet at the moment
Jekka MacVicar - herbs, herbs and more herbs
Sue Beesley - winner of BBC Gardener of the Year some time ago now. Now a professional nursery owner. Good writer. Active on Twitter as well.
Veg Plotting - amateur based in Chippenham. Often interesting
Let us know if you find anything interesting