These are apps that I use on all my systems starting with Firefox Web Browser! Then in no particular order….

Game Booster Turns off unnecessary services before gaming! Even when you have a uber fast rig – sometimes certain services can slow certain games down.
Case in point – for me Far Cry 2 lags until I shut down Pen Tablet Services that I use for my Wacom Bamboo tablet.

Crap Cleaner CCleaner is a freeware system optimization, privacy and cleaning tool.

Smart Defrag™ The Automated Defragmenter, quick and easy to use.

SyncToy v2.0 Local File Backup.  I set mine to Echo and back up my files to my WD My Book external drive every week or so.

Process Explorer Find out what processes are running in the background on your PC.

Windows Live Essentials Now in one installation, Windows Live Essentials gives you instant messaging, e-mail, blogging, photos, and more.

Microsoft Security Essentials Microsoft Security Essentials provides real-time protection for your home PC that guards against viruses, spyware, and other malicious software. 

Zune – Media player I especially use for subscribing to Podcasts.