See all settings. Click the Accounts and import tab. In the "Check mail from other accounts" section, click Add a mail account. Type the email address of the other account, then click Next. Make a selection and click Next.

It isn't currently possible to merge separate Google Accounts. However, if you'd like to transfer your data from one account to another, this may be done on a per product basis. Or, to start using a new product, you don't have to create another Google Account.