Gmail's latest Labs creation is one of the best yet: panes that let you create separate inboxes using filters and labels. Anal-retentive over-organisers of the world, rejoice!
Want to have a pane just for emails from your family? Or from a particular mailing list? You can easily split up threads from different people or groups or based on different subjects or anything you want. It's a great way to use filters to turn one email account into many. Brilliant.