The official homepage for xxdiff is now http://furius.ca/xxdiff. You should be redirected in 8 seconds. If you are not redirected automatically, click on the link above.
I--the author, Martin Blais--was getting annoyed with maintaining two web pages up-to-date at the same time and therefore I have decided to rationalize all my projects on my own server, where everything gets updated automatically due to some scripting wizardry. Don't worry, this project is still alive and kicking (and it's mature and stable).
If my server is down momentarily, you can always view a copy of the possibly outdated pages at SourceForge.