OpenOffice.org

This is a suite of applications:
• Writer (wordprocessing)
• Impress (slide shows / onscreen presentations)
• Calc (spreadsheets)
• Math (mathematical functions)
• Draw (graphics and diagrams)
• Base (databases)
You have the option to install just the components that you will use. OpenOffice.org has its own default file formats (e.g. .odt, .ods), but popular file formats like .rtf, .doc, and .xls are also supported.

Just want wordprocessing? Give Abiword a try.

Scribus

Scribus is a desktop publishing program that you can use to create newsletters, flyers, PDFs, etc.

the GIMP

The GIMP is a program for working with images - photo editing and/or graphic design.

If vector images are more your thing, you can try Inkscape.

Mozilla Firefox

You might already be using it! Firefox is the Internet browser that introduced tabbed browsing and browser extensions.

Pidgin

Pidgin is an instant messaging program that supports Yahoo!, MSN, ICQ, AIM, MySpace, and other IM protocols, so that you can log into two different accounts at once.

Kompozer

Kompozer is a Frontpage-like WYSIWIG program to help you design webpages.

VLC

VLC is a media player with support for many kinds of audio and video formats, plus DVD, VCD, and streaming formats

Miro

Miro is a video and Internet TV player, which includes a feature for searching and downloading YouTube videos from within the player itself

Battle for Wesnoth

When you're on a break, you can try Battle for Wesnoth, a strategy game between different races and creatures in another world.

A list of more games - RPGs, shooters, racing, scrolling, and puzzles - can be found here and here.