Google is rolling out the much hoped-for capability of being able to sign into multiple accounts in the same browser.
According to a post on the unofficial Google Operating System blog, a Multiple Sign-In option is beginning to appear in the Personal Settings area of your Google Accounts page — and if it's not there yet (ours isn't), it will be soon.
This upgrade — which began to be tested just a few weeks ago — will be most welcome, perhaps, to users with multiple Gmail accounts, but other Google products are now capable of multi-account access, and more will be added over time.
In addition to Gmail, you can use multiple sign-in with the desktop, but not mobile, versions of some, but not all, Google products: App Engine, Code, Calendar, Reader, Sites, and Voice. Docs is listed as "coming soon". Other Googly tools and all mobile products will instead default to the first account with which you signed in.
There is, however, a workaround to non-supported Google products: the tried and true Chrome Incognito cookie-ignoring mode. Alternatively, of course, you can use multiple browsers or IE8's "new session" capability.
The multiple sign-in capability is still very much a work in progress, albeit a welcome one. A list of known issues includes the inability to use offline calendars or mail, problems with some Google gadgets, and more.
You can find more info on Google's Getting Started: Enabling Multiple Sign-In help page. ®