I would like to add my site to this list if I may. I didn't want to go and spam the main forums, but saw this thread and found it very appropriate.
Kinfinity (formally known as allKindle) is a UK specific Kindle forum. As much as MobileRead and KindleBoards are excellent resources, they are very USA-centric, mostly because the Kindle 3 is the first incarnation to be "officially" in the UK. allKindle was set up to fill that particular gap in the market, and to compliment all the sites are already there. More choice is a good thing IMO.
With over 3600 posts and almost 350 members since it launched 6 weeks ago, I hope allKindle can be considered a link every Kindle owner should have.