CheatSheet is lets you view available keyboard shortcuts in your current application. Just hold the ⌘-Key a bit longer to get a list of all active short cuts of the current application. It's as simple as that.
Alfred is an award-winning app for Mac OS X which boosts your efficiency with hotkeys, keywords, text expansion and more. Search your Mac and the web, and be more productive with custom actions to control your Mac.
Power Menu is a highly configurable Finder extension that adds powerful actions to the right click menu and toolbar within Finder - this makes it quick and easy to do things such as move and copy files, convert images, create new documents and much more. The app comes bundled with preset actions and supports custom shell scripts.