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.
Pretzel is Mac desktop app that shows and find keyboard shortcuts based on your current app.
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.