Gmail labels are very nice and they work great with filters. However, applying a label manually is cumbersome. Labels are kind of hidden in the "more actions" menu, the user has the use the mouse to do do it.
Mailplane now makes applying and removing a label much easier. Everything can be done from the keyboard; no mouse interaction is required.
Either press Cmd-L (or just the "L") key and open the label auto completion. Then narrow down the list by entering a few letters, press the enter key to apply or remove the label.
Watch screencast to get started:
Important: This feature works in Gmail 2 only!