Note to webapp developers: the + character is a valid part of an email address. Don't stop me from using it. And fix your broken CGI parsing so + isn't treated as space.
The + has a very useful meaning in almost all mailers: stuff after it is ignored. Ie, email@example.com and firstname.lastname@example.org are basically treated as the same email address by example.com's mailer. Both addresses deliver to the mailbox owned by xyzzy. This has been a feature of sendmail for at least 10 years. It works in Postfix and qmail, too.
The plus sign makes it easier to track how your email address is spread around. If I ever get email to email@example.com, I know who to blame. It's also helpful if you're debugging webapps and need to create email-keyed accounts.