Use the advanced search operators to build searches like label:name-of-label, subject:linux or is:unread -in:inbox. If you were expecting to actually see the inboxes of multiple email accounts, you can achieve this by using Gmail's mail fetcher, which automatically labels the messages fetched from other accounts and archives them. You can then create panes that have the following format: label:myotheraccount@gmail.com or to:myotheraccount.com.
"After you turn on Multiple Inboxes from the Labs tab under Settings, you can configure what you want to see, as well as set the number of messages displayed and the positioning of your panels from the Multiple Inboxes section under Settings," explains Octavian Costache, who created this feature.
"Multiple inboxes" works especially well if you have a high resolution monitor and if you use filters that automatically archive some classes of messages.
Related:
Simplified Gmail searches
A dashboard for Google Docs
