Gmail Lets You Set From: Header
Gmail now lets you configure the From: header of the mails you send. This is great news if your “official” mail address is something other than “…@gmail.com” and you only use Gmail as your back end for storing, reading and writing mails. With the proper From: header (as opposed to the Reply-To: header, which has always been configurable), people won’t accidentally add your Gmail address to their address books. This in turn means that it is easier to switch to a different mail system if you should become unsatisfied with Gmail.
This is a pretty generous feature from Google, considering that they are really removing a lot of lock-in into their system.
Updated: Added screen shot:

Update: As Paul points out in the comments below, Your Milage May Vary depending on the user interface language set in Gmail. Its seems, though, that you can switch to English (US), add a new “From:” address, switch back to your favorite language and still use the new address.
September 29th, 2005 at 7:41
But the facility to modify “From:” seems to be removed. Or I cannot find it.
September 30th, 2005 at 6:54
It’s in “Settings / Accounts / Add another email address”. See screen shot.
September 30th, 2005 at 7:48
That is not the screen I see! The 2nd tab is not “Accounts” - that’s missing.
September 30th, 2005 at 8:02
Haha! I read on the web: Your interface language must be US-English. I had it at UK-English. Try it yourself - different interface!
October 1st, 2005 at 9:44
Interesting. I tried two other languages (German and Japanese) and the “Accounts” tab is missing there, too.
November 25th, 2005 at 22:00
Thanks hugely for the US language tip! The missing ’settings’ tab was driving me crazy! Would never have guessed that was the problem!