Chrome: It's ridiculous that you have to open multiple Chrome profiles or incognito windows just to sign into different accounts on the same web site. To quickly log in with two or more profiles for one service, it doesn't get easier than Multi-Account Login.
The Chrome extension works a bit like Multifox for Firefox, but I actually found it even simpler to use. It appears as a little button in your toolbar. Click and it opens a new tab that acts like an 'incognito tab' -- which was originally this extension's name. You can now visit any web site in this tab, and whether you are already signed into the service in another tab or not, you can start another session with a different login right here.
It also adds an option in the right-click context menu to "Open Page in New Account Tab", duplicating the web site you are currently on. There's a number that appears as a permanent prefix to the tab and there's no way to remove it, but hey, it's a small price to pay for convenience.
Multi-Account Login [Chrome Web Store]