Here are a few useful free Apps
Click on the icons to visit App website

Mactracker

Mactracker provides detailed information on every Apple Macintosh computer ever made, including items such as processor speed, memory, optical drives, graphic cards, supported Mac OS versions, and expansion options.
Appzapper

Everybody loves the drag and drop nature of OS X. Drag an app into your applications folder, and it's installed. You'd think it would be that easy to delete an app — just a matter of dragging it to the trash. But it's not. Apps install support files on your computer that generate clutter. Introducing AppZapper. Simply drag one or more apps onto AppZapper. Then, watch as it finds the extra files and lets you delete them with one click.
Perian

Perian is a free, open source QuickTime component that adds native support for many popular video formats.
Dropbox

Dropbox is software that syncs your files online and across your computers.
Put your files into your Dropbox on one computer, and they'll be instantly available on any of your other computers that you've installed Dropbox on (Windows, Mac, and Linux too!) Because a copy of your files are stored on Dropbox's secure servers, you can also access them from any computer or mobile device using the Dropbox website.
Evernote

Evernote makes it easy to remember things big and small from your notable life using your computer, phone, and the web. Get started today with a free account.