Many bug tracking —and some project management— web-apps offer Subversion integration. Trac and some other bug trackers can scan your commit messages for snippets of text that refer to ticket IDs. If you, for instance, commit a change with the text closes #25 in your commit message, ticket 25 in your Trac instance will be closed and a comment is added to the ticket that refers to the new revision.
Thanks to Subversion's repository hooks and other similar functionaility, you can hook up pretty much anything to Subversion. There are several pages in the SVN Book that can help you get started.