A few community sites I actually hang around and use - although some I only found recently so not sure how long I will use them (updated March 25)

LibraryThing.com - i tried shelfari and although it looks pretty it just didn’t click - i started adding a few books but it made it slower than practical, and it didn’t really seem to be aimed at the book lover. I had been hearing about librarything via some blogs i read, so i figured I’d try that to compare. Before I knew it i had 500 books listed, by just adding a few books from 5 authors then and adding all the ones I own which popped up in the related items. I even have ordered the little barcode scanner to add the boxes of books reliably. http://www.librarything.com/catalog.php?view=iphigenie

LT is accessible via mobiles and has saved me from buying duplicates a few times already

Via LT I found the following websites:
What’s that book? http://www.whatsthatbook.com/ - a cool little “help me identify this book” forum

book swapping http://bookmooch.com/. I have sent over 100 books now via bookmooch already, and requested almost as many.