Coming soon from Google!
Now here! This feature was incorporated into Gmail itself after the other answers were written:
Now, you can visit google.com/accounts and click the link next to "Multiple sign-in." After you sign into your first account, you can sign in with up to two additional accounts from the new accounts menu in the upper right hand corner of Gmail, then easily toggle back and forth between them. You can even open multiple Gmail tabs — one for each of your accounts.
