free for X

Adium

I'm not much of a chatter, but if I did chat I'd use iChat. If that didn't work, Adium seems like the next best choice -- given all the praise it has received.

Audion

This alternative to iTunes was recently retired. You may find it more to your liking than Apple's product.

Audacity

It records and edits sounds with aplomb, with tons of new features constantly being added.

Genius

This is great little educational tool for memorizing the facts.

Meteorologist

Check out the current weather and forecasts from the menu bar or the dock. There is also a free version of WeatherPop, but it only covers U.S. cities.

Quicksilver

This is a cool way of finding and launching files and applictions. Unfortunately it didn't always work for me, but you may have better luck.

Spark

If you're into hot keys, you'll be hot for Spark. It's not all that easy to get set up, but you could very well find it worth the effort.

Stellarium

Gaze with wonder at the night sky constellations from wherever you happen to live on this planet. Then go outside and see the stars as they are. Or if you want to check out planet Earth from the heavens, try Celestia.

VLC Player

It will play just about any kind of video or audio format you can imagine -- thus making it one of the most popular downloads of all.

wClock

Ever wonder why the menu bar doesn't display both the day and date? So have I. And it looks like Wolfware not only wondered, but came up with a solution. Speaking of menu calendars, you also might want to try MenuCalendarClock.

Current favourite

What a beautiful way to explore the universe.