Changing the brightness with Macbook keyboard always changes the laptop display. External keyboard always changes external display (if attached). Ctrl + brightness key inverts the behavior for both external and internal.
Sloppy desktop habits? Too many GIFs laying around in ~/Pictures? Select a bunch of files and throw them in a new folder with two clicks.
If you save a lot of drafts in Mail, and especially if you have multiple accounts, you don’t have to go digging for your account’s Drafts mailbox to start editing again.
A handy $3 utility for creating simple, visual alerts has a system-wide shortcut, Automator support, and even a URL scheme if you really want to start cooking with apps like Alfred and LaunchBar.
Especially handy since MarsEdit works with a bunch of services like Tumblr, WordPress, and Blogger.