Gmail has an interesting characteristic that falls somewhere between “really cool feature” and “strange parsing bug”, depending on how you want to look at it. Whatever the underlying cause, the end result is that you can create custom aliases to your Gmail account that can be used to help differentiate your accounts. There are two different ways. The first one is the appended alias.
The second behavior, which I believe is unintentional, is delimiter invariance. In most e-mail systems, “FirstLast@gmail.com” and “First.Last@gmail.com” are treated as unique identifiers. They may be bound to the account on the back end. Gmail appears to use complete delimiter invariance in their systems, though. My e-mail address does not currently contain any separators, but if I sent e-mail to first.name, f.irstname, firstnam.e, or even email@example.com they all route to my primary e-mail account. This is especially interesting because another Internet user in another part of the country with a similar name, has registered the same e-mail address. Gmail allows periods when creating the account, but ignores them. I occasionally get e-mail directed to this other individual because of this issue.
Combining these two, you can sort and filter your e-mail a bit before it even reaches your inbox by making your address uniquely identifiable. It’s a pretty neat feature.