Audicity - Free OS X MP3 editor

Looking for an application that can edit your MP3 files? Want to move that bonus track into its own file and cut the 5 minutes of silence? Look no further. Audacity will do this for you, for free. This audio file editor is loaded with functions, but you'll probably be using Trim and Cut most often. And once you figure out how, that works perfectly. For export to mp3 is relies on Lame. This is the de-facto standard for mp3 encoding, but because of licensing, unfortunately it does mean an additional download.
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download Audicity - Free OS X MP3 editor

iPodDisk

If you own one or more iPods, you will desperately need this tool at some stage. iPodDisk allows you to browse the files on your iPod, just like a normal drive. That way you can copy all your music, videos etc from your iPod to your mac. Since iTunes only allows you to sync one way (from mac to iPod), this tool is a lifesaver if you happen to loose files on your mac and you know you still have them on your iPod. Simple tool, but works very well. Note that installation is a two step process. Download and install MacFuse (a file-system utility) first, before running iPodDisk.

download iPodDisk

SketchUp

3D modelling used to be an advanced technology for which you needed complex and expensive autocad programs. But now the good folk at Google have made it available to the masses. SketchUp is a smart little tool that allows you to easily create 3D drawings. The paid-for Pro version will give you all functionality, but the free version will be quite enough to make a nice drawing of that desk you were going to build.
not open-source, but it is free

download SketchUp

Transmission

If you're into file-sharing and p2p, you will need a good BitTorrent client. BitTorrent
have their own free Mac client, but somehow this feels better and quicker.

download Transmission

The Unarchiver

You can live a year without ever needing this application and then all of a sudden you desperately need it. The build in OS X unarchiver will do most jobs, but for things it doesn't handle well, like multifile / split RAR archives, this is a perfect little tool. Nicely integrated in OSX as well.

download The Unarchiver

Miro / Democracy

More than just a video player. Miro also does Internet TV (somewhat similar to Joost), YouTube search and download, and it provides BitTorrent support. The preloaded channels are not great (yet), but it's easy enough to add any RSS video channel. This application used to be called Democracy.

download Miro / Democracy

Mozilla Thunderbird

From the same group that's behind Firefox, here's an email client that's much more powerful than Mac Mail. It's not as nicely integrated with iLife though.

download Mozilla Thunderbird

Adium

Tired of having to open all those different IM applications just because you're friends can't agree on the best one? Upgrade to an all-in-one Instant Messaging client. Adium connects to AIM, MSN, Jabber, Yahoo, GoogleTalk and many more.

download Adium

VLC

Much more versatile than QuickTime. If a video file doesn't play or if you can't hear sound, then try this player. Does full-screen play-back as well. Supports AVI, DIVX, OGG and many more formats.

download VLC

Seashore

You can avoid all the complexity of Gimp by opting for Seashore. Perhaps not as powerful as Gimp, but a lot easier to use and it will handle most of your daily editing tasks quite well.

download Seashore

Gimp

A little complicated to install, as you will need to retrieve your Mac OS X Install Discs to install X11 first. Then every time you run Gimp, it will open the X window system first. Sounds complex? It is, but in return you get an extremely powerful image editor, very similar to Adobe Photoshop.

download Gimp

KompoZer

iWeb is nice, but only really works well if you use it with a .Mac account. Dreamweaver is good, but can be complicated and is expensive. The open-source alternative has always been Nvu. Unfortunately the developer started working on something else. The good people from KompoZer have taken the code-base, fixed some bugs and added useful features like CSS support.

download KompoZer

Mozilla Firefox

Even if you love Safari, you will still need firefox. It may not be as pretty, but more sites will work properly. And with all the additional extensions you can make it do almost anything.

download Mozilla Firefox