The latest addition to ubuntuupdates.org is the PPAs page. Personal Package Archives are repositories that are not supported in any way by Canonical, but that offer versions of packages that are up to 6 months ahead of the official repositories.
It's sometimes frustrating to have to wait for that long for major new versions of the most popular applications. Take Thunderbird 3.0 for example, it was officially released by Mozilla in December, 2 months after Karmic but is not and will not be part of any official Karmic repository. However it's in the Mozilla Daily Builds PPA and you can install it from there.
As before, UbuntuUpdates.org lets you list all versions of any package in any release. But now, versions of the same package from PPAs are also listed. For example, the MythTV package, part of the Mythbuntu PPA, is shown here alongside official repo versions. The PPA page itself shows the list of member packages and explains how to setup the PPA.
UbuntuUpdates.org doesn't track all existing PPAs (there are thousands, see official launchpad.net PPA website), it only tracks those that appear to be the most popular. If there is a PPA you'd like to see here, leave a comment and I might add it as long as it's not too exotic.