Good idea. For those of us who use an @gmail.com address, here's what you can use in lieu of plus signs, which as JimmyL said often don't work:

Gmail lets you insert periods between the letters of your username. So if my email address is someusername@gmail.com, the following are valid variants of my email address:

some.username@gmail.com

some.user.name@gmail.com

s.omeusername@gmail.com

And so on. So you can use variants for different services you sign up for. Also note that you can substitute @googlemail.com for @gmail.com.