Much to my delight, GMail has added an ‘Activity on this account’ feature. It is located down at the bottom of the inbox page, where it lists the time of last account activities. Clicking ‘Details’ leads to a pop-up showing the last five instances of account access, the form of access (browser, POP, IMAP, etc), and the IP address.
This is a big security advance. Previously, anyone who knew your GMail password could access your account at will, with no way for you to know. They could even be logged in at the same time as you, with no sign on your machine that this was happening. This is also addressed by the new feature, which includes an option to log out all other accounts.
GMail users should definitely take a peek at this information from time to time, especially if they are in the habit of using their account from shared or public computers. Given (a) how much information the accounts store and (b) how easily searchable they are, any attack that gains access to your GMail account could have serious consequences.