Say what you like about Catholics (and I sometimes have) - this is something we need around here.
Penance is about remediation of the harm you have caused and teaching yourself to do better in the future.
I humbly and publicly confess that I was the AnonymousCoward who accused KeithBraithwaite of being ironic and thus English in an early, unrefactored fragment of ImaJavaProgrammer. I thus directly caused the long historical rant in EnglishOrBritish. Have mercy and please duly refactor countless previous offences in laying down my WikiPenance. As I now recite the WikiPrayer... -- AnonymousCoward
Every edit I do as a WikiGnome is penance for my non-gnome contributions.
One of the nicer rules about penance, at least in Christianity, is that you're not allowed to point out the sins of others. You must mind your own sins and they must mind theirs. Repentance has to come from within. You can't tell someone to repent. That's just not in the spirit of true penance.
See also: WikiSinRank