Somehow managed to commit something under the wrong username? Want to know how to change it? It took me some googling to find out how, so that's why I put everything I found on one page.
$ svn propset --revprop -r revision_number svn:author your_username
You might get something like this:
svn: Repository has not been enabled to accept revision propchanges; ask the administrator to create a pre-revprop-change hook
To solve that you need to be administrator of the svn repo: if you're not, you can't follow the next steps.
Go to the 'hooks' directory in your repository (eg. /var/svn/my_repo/hooks/), copy pre-revprop-change.tmpl to pre-revprop-change, edit the pre-revprop-change file to make sure setting the author is enabled (or see below), make the script executable (chmod +x pre-revprop-change) and you're done.
If you can't figure out how to change the pre-revprop-change file to allow authors, just creating a new file with the name pre-revprop-change and the following contents should work:
#!/bin/sh
Don't forget the chmod step though.
If you're using trac, make sure you use trac-admin resync after the svn propset.