Google has a "multiple sign in" setting that will allow you to sign in to more than one account using the same browser session.

Go to this page for instructions on set up, use and limitations:

http://www.google.com/support/accounts/bin/answer.py?answer=181599&hl=en

You may also find it convenient to use different browsers for different Google accounts.

Finally, some browser extensions help to manage different Google accounts. Here is one for Firefox:

http://www.longfocus.com/firefox/gmanager/