Google has announced a multiple account sign-in feature that makes it easier for users with more than one Google account to manage their online identities. Google allows using three Google accounts at one time with the multiple sign-in feature at present
At the moment, you can only use multiple Google account sign-in to use App Engine, Google Code, Google Calendar, Gmail, Google Reader, Sites and Voice. Multiple account sign-in for Google Docs is not ready yet, but will be coming soon.
More info: http://www.google.com/support/accounts/bin/answer.py?answer=181602
