Llewtrah's web site contains a large number of bawdy parodies as well as rugby songs. They are mostly a bit too crude for my taste though.

Don't have the URL to hand, and the automatic censor probably wouldn't let me find it here! But there are links to it in other threads.

I once heard someone sing 'The BOY from Ipanema'. Only changed a few words, but the whole song became totally different...