1. You can append a string to your Gmail account with a plus sign. This will automatically route the mail through to that name to your account — which you can then see (and filter on).
So, for example, if you put in an e-mail address on a donation to Group X, you can use mygoogleid+groupx@gmail.com. The mail then comes in to the mygoogleid account, but with that suffix. I can see if Group X sells my e-mail to anyone, I can easily filter (or auto-delete) all the mail that comes in to that address, etc.
2. You can put all the dots in your name (or not) that you want in Gmail and they’ll be ignored. So if you have fredsmith as an ID, you can use fred.smith or fredsmith or f.reds.mith and the mail will go through. This could be handy both for you as the account owner (to put that in public displays of your account name so that people can more easily read it), or you as someone sending to a Gmail account (so that you can more easily read it).
3. You can set up an online chat link on your blog, web page, etc., with Google Talk. This might not be helpful for blogging, but could be for business. The user doesn’t need a Google Talk account, and it will reflect your on/offline/busy status.
1. Damn I wish I knew about this 4 years ago! This is by far the greatest feature of gmail.