Regular Gmail users now able to delegate account access
The option is already available, and can be found in Gmail Settings, under the "Accounts and Import" tab. When you enter the Gmail address of another user as an account you'd like to allow, that user will receive an automated message with the option to accept or deny the invitation. Once accepted, they will be able to sign into your account from their own inbox using a small drop-down triangle next to their address in the upper-right corner of the Gmail homepage. Each account gets its own tab (or browser window, if you still roll like that), and whenever a delegate sends a message on behalf of your account, the "from" field of that message will note the fact -- so your delegates can never screw you over by acting like they're actually you.
It's all kept nice and simple, but there's one catch: If that user signs out of your account, they sign out of their own as well. Granted, it's really a trivial gripe, but it could get annoying over time. It's a bit on the dry side (read: It's incredibly slow and boring), but you can watch Google's original Apps demo video for this feature after the break.