There are thousands of excellent open source programs to choose from. Here are 10 of my favorites that rarely get breathless buzz or make top 10 lists, presented in no particular order, rhyme, or reason.
Ardour Digital Audio Workstation (DAW)
The Ardour digital audio workstation is a first-rate multi-channel audio recorder, editor, and mixer. It runs on Mac OS X and Linux, which are both superior audio production platforms. It supports a wide range of audio file formats, has non-destructive editing with unlimited undo, and unlimited tracks and buses. (Limited only by your computer when it reaches its limits and keels over.)
Ardour organizes a recording session by buses. An audio bus, whether hardware or software, is a grouping of tracks. Then any edits are applied to all tracks on the bus, as though it were a single track. For example you might route all the bassoon tracks to a single bus because you want to get editing the bassoons over with quickly. If you prefer to put every voice and instrument on its own track, then your Ardour buses will have one track each. Ardour supports all manner of special effects plugins, has good video synchronization, latency correction, networked audio, and lots more. (GPL, Linux, Mac OS X.)
Orca Screen Reader
The computer world hasn't paid enough attention to accessibility for vision, hearing, or physically-impaired computer users, so there is only a small selection of applications. One of the best is The Orca screen reader. Orca has screen magnification, text-to-speech with multiple speech synthesizers with configurable pitch and speed, male and female voices, and it supports Braille displays. Braille displays are little hardware gadgets that replace the computer monitor. Screen output is translated into Braille by Orca and sent to the Braille display, which is like a keyboard, but with 40 or 80 Braille cells instead of QWERTY keys. Each cell is a dynamic Braille character that refreshes as the text changes, so people who read Braille can read them like a paper Braille document.
Braille displays also support navigation functions like page up and down, scrolling, paging, and keyboard shortcuts.
Orca supports the assistive technology service provider interface (AT-SPI), which is the assistive technology infrastructure for Linux and Solaris. The Gnome desktop supports AT-SPI, and so do key applications like Firefox and OpenOffice. An easy way to see this in action is to download Vinux, the Linux distribution specialized for vision-impaired users, and run it from the live CD.(GPL, Linux.)
MuseScore Score Writer
MuseScore is an elegant and efficient music notation editor, for writing and printing music scores. It supports MIDI and MusicXML for interoperability with most other score writers, has built-in playback, and fast note-editing. It outputs to the WAV, Ogg, or FLAC audio formats, PDF, SVG, PNG formats for printing, or to the LilyPond score engraver for additional prettification. It is popular in music education programs all over the world. (GPL, Linux, Mac, Windows.)
Dnsmasq Name Server
Dnsmasq is one of those reliable, simple, useful servers that does what you ask with no backtalk. It is designed to provide DHCP, DNS, and TFTP services to LANs and home networks. It is not an authoritative DNS server, but provides local name services from a single etc/hosts file. It includes a caching resolver, a DHCP server, and it does all this in a single simple configuration file. (GPL, Linux, Unix-type operating systems.)
Kajongg, Real Mahjong
Kajongg is one of the few real Mahjong games for any computing platform, and I think it is the most beautiful. It is not one of the common solitaire tile-matching games, but a proper four-player competitive game. Mahjong is meant to be a social game, and often a gambling game, and a good game with skilled players can take an hour or more. You can play any combination of humans and computer players, and setting up a game server for multiple human players is fast and easy. Kajongg comes with a decent manual, but if you are new to the game you will probably want your own rulebook. (GPL, Linux.)
LibreOffice Productivity Suite
LibreOffice is the shiny new fork of the popular OpenOffice productivity suite. When Oracle bought Sun Microsystems they acquired OpenOffice, and then abandoned it. OpenOffice's future is uncertain, but LibreOffice's is not. LibreOffice is a welcome breath of fresh air and new energy, and already is better than its ancestor. Don't waste gobs of money on overpriced office suites; LibreOffice comes with everything you need, and unlike its spendy proprietary brethren is friendly and open: open code, open document formats, cross-platform, and translated into many languages. (LGPL, Linux, Mac, Windows.)
Game Editor Game Creator
Want to create your own 2D games for PCs and mobile devices? Try Game Editor, the cross-platform royalty-free game creator designed for beginners. Turn your ideas into games on any platform for any platform: Linux, Mac, iPhone, iPad, Windows mobile and PC, and more. (GPL, most platforms.)