FranticFish - 2:40 pm on Sep 24, 2010 (gmt 0)
BackLinkWatch and all the free tools I've seen are based on Yahoo's dataset. All that's different is the GUI and extra features (a bulk PR checker, showing the anchor text, indicating nofollow etc).
Google Webmasters is way out - has been for a while.
Someone mentioned Majestic. Best I've seen and free for your own site(s).