If you use two or more Gmail accounts, you can stay logged in with both in one browser by switching on the multiple login option: Log in, go to Account Settings and click the Change link under Multiple Sign-in.
After the consequent setup, you can open multiple tabs for multiple logins, or simply Switch account from the top menu. The same goes for any services linked to Gmail, such as Google Docs, Reader, etc.
Inevitably, you will want to change the default account: as of writing, the default account is the one you logged in with first and there is no option to lock an account as default. The only way to change your default is to logout from all accounts, and login with the one you want to serve as the new default.

