Google Chrome's "Incognito Mode" isn't just great for hiding your sultry late night browsing habits, it can also keep you logged into the same webapp as a different user than your main window, like two Gmail accounts. For those of you unfamiliar with Incognito Mode:
Web pages that you open and files downloaded while you are incognito won't be logged in your browsing and download histories; all new cookies are deleted after you close the incognito window. You can browse normally and in incognito mode at the same time by using separate windows.
Because of the lack of cookie crossover between your standard browsing window and the incognito window you can easily log into two accounts within the same service, a feat not possible with the standard browser model. Get more advanced Google Chrome tips here. Not a Chrome early adopter? Use the Cookie Pie Firefox extension to control cookies at the tab level and achieve the same multiple login magic.