I wish I had known about this before I went to TechEd two years ago - I'm still getting email from vendors who scanned my badge there…
Gmail ignores periods in user names and doesn't care about capitalization. (This is not true of Google Apps - It does care about dots.)
If your email address is homerjsimpson@gmail.com, all of these variations will be delivered to your Inbox:
You can create a filter to perform actions (forward, trash, apply a label, etc.) based on the "To" address. In this example, I'm applying a "might be spam" label to messages addressed to "homer.j.simpson":
If you're filling out a form for a site where you don't trust that they won't
"share your email address with trusted partners who will send you special offers and useful information":
(
)
…you might want to give them a "dotty" email address so can have more control over the messages you receive.