Have two Gmail accounts that you want to keep open at the same time in the same browser? Check out CookiePie, a simple browser extension that will fool your browser's cookie manager into thinking you've got one account open. Once installed, you can have several instances of Gmail, Google Docs, or any other service that doesn't let you have two windows or tabs open with different accounts.
What makes the extension nice is that you can toggle it on and off with a simple right-click on any tab. My test with Gmail got me to the point of login on a secondary account, but actually trying to open and create messages gave me a dreaded Gmail error message. The same went for Google Docs and Spreadsheets, so there's some work to be done--at least with Firefox 3, that is. Your mileage may vary on earlier versions, and will probably be fixed in the coming days. Still, this looks to be a handy tool for folks looking to save time by having multiple accounts open without having to resort to the browser shuffle.
Embedded below is a video of it in action. You can grab the extension here.