with gmail you can do:

    foo+anything  => redirected to foo
    foo.something => redirected to foosomething (so . is not the same as +)

The + isn't always accepted in dumb email checks though, and spammers know about it...

I believe you can insert dots as you wish but not use it like the plus sign. So abc@gmail.com is the same as a.b.c@gmail.com but abc.new@gmail.com is another account than abc@gmail.com.