Here's a really amazing new feature from Google -- you can now open 2 GMail accounts at the same time on two different browser tabs/sessions! Previously, if you login using a different account, it will logout your other account first.
I have multiple GMail accounts: one for personal, another one for ThePictureShop, and another one for Dragonpay Online Payment System. Its very tedious having to logout/login to switch from one account to another, so I end up just forwarding everything to my main personal account and answering my email from there. The problem with doing it this way is the reply-to address is my personal email.
By enabling the Multiple Accounts option in your Google Accounts page, switching from one account to another becomes very easy. You just pick the account you want from the dropdown list at the upper right corner.
The system is not available for all Google services as of now. Picasa and Blogger will only work with the main login account. But that's fine with me. I really just need the multiple login capability on GMail anyway.