[k-mcgrady]

Pretty sure you can use two regular windows to run separate accounts (although the colour coding would be useful). Another option is running multiple user profiles in Chrome.

[pearkes]

Cookies are shared between standard windows (unless there's a way to disable that).

Haven't looked into the profiles. Speed of switching would be the most important thing for me.

[kylemaxwell]

Well, if you use incognito mode, you can do this. Doesn't scale past n=2 but helpful for many use cases.

[infinite8s]

Especially for facebook, which can still track you across other websites even if you log-off.