Firefox
The first app I install, Firefox is world's most popular alternative browser. I love this browser for its look and feel, speed, addons and tons of features!

Foxit Reader
Foxit is a PDF reader you might have never heard of. Most people will use Adobe's PDF reader, since thats the one most people only know about. I hate Adobe's PDF reader, it has loads of great features, but it is very slow and heavy on system resources. Foxit Reader is designed to lightweight, very quick to load, and just provide the features to view the PDF document.

OpenOffice.org
Microsoft Office is an excellent set of applications, however it can very expensive to buy. If you need an office suite, OpenOffice.org is a free office suite which is very similar to Microsoft Office. It has a word processer, spreadsheet, presentation, database and drawing. Plus, OpenOffice.org can open Microsoft Word, Excel and PowerPoint files. OpenOffice has about 90% of the features which Microsoft Office has, and I believe the interface in OpenOffice is better.

VLC
VLC is a media player, and I love it, just because it will play any audio and video file format you throw at it!

CCleaner
Windows computers can get full of junk files, old applications and a messy register. CCleaner is an excellent tool for removing junk and cached files, which can save you 100s of MBs of data. It can safely clean out your registry, which can become messy over time. It is also a very good tool at uninstalling applications and managing the startup of applications in the background.

Pidgin
If you need a lightweight Instant Messenger (IM) client which can handle multiple IM accounts in one application, Pidgin is for you. There is no need to have MSN messenger for your MSN contacts and Yahoo Messenger for your Yahoo contacts, etc; you can have all your contacts in one application! It supports over 20 accounts from different providers. Plus it has great features such as tabbed chat windows, video/audio chat and file transfer.

GIMP
Adobe Photoshop is the hallmark in photo editing, however it is incredible expensive. If you need a photo/graphic editor for free, with the majority of the same features as Photoshop, GIMP is what you need!