Gmail offers a handy feature for adding email address aliases by appending a '+' and a term to the end of your email address. See Using an address alias and Wikipedia:Sub-addressing
The issue I keep running into is incorrect email address validation preventing the use of characters that should be valid according to RFC 5322 and RFC 3696.
What annoys me most is that there often isn't an easy way to tell a site that there is a bug. Some problematic sites I've come across:
From some quick Googling it may be that RegEx just won't cut it for email address validation.