Have you ever wanted to open multiple gmail accounts or use other Google services all at the same time for testing purposes? In the old days there was CookiePie for firefox (which never really worked with Gmail). Just use Google Chrome's Incognito mode by pressing Ctrl-Shift-N. That is it! Your original (aka non-incognito, non-InPrivate, non-Private-Browsing) windows will continue logged in as normal when the Incongito windows are closed. It seems that each Incognito window shares cookies between that windows tabs, but not among any other windows, Incognito or not. Make sense? You can open as many different copies as you want up to your amount of memory, since nothing is ever written to disk, but rather is stored in memory.
This should also work with any other service that doesn't tie logins to IP addresses, but to cookies like Facebook, Mypsace, etc.... I don't really know if it works with IE8's In-Private browsing because I'm allergic to Microsoft software, and use it only when forced to.
Be careful composing in Blogger or using another service though, as nothing is cached at all. If you go back a post that wasn't saved as a draft will be gone.
Update (April 23, 2009): It seems that you can only login to two Google accounts at a time, one in an incognito window, and one in the regular window. Also it seems that playing Windows Media Player videos in Incognito mode leaves traces in Internet Explorer!