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At work I use a Mac Mini running OS X, and being an open source person I try to use as much open source software on my Mac as possible. So when I got it in the beginning, I looked for open source applications for all the various things I wanted and needed.
Finding Open Source Software
One of the difficulties I encountered when looking for OSS for Mac OS X was that there isn't a huge amount of it. It seems that most software for Macs is commercial. Fortunately I found a site that is a great help: Open Source Mac. This site lists a lot of open source software for Mac OS X, and I use a number of applications that are listed there.
However, not all software is listed on Open Source Mac. I couldn't find a decent audio player. Mac OS X comes with iTunes, but iTunes doesn't support Ogg Vorbis files, the format all my music is in already. Eventually I found a little discontinued app called Audion, which works fairly well (although it hangs occasionally and cannot handle complex audio) and, most importantly, supports Ogg Vorbis files.
Before I babble on much further, let me list the open source applications I found and that I use on a daily basis.
Web Browsing: Firefox
Of course the first thing I did was to download Firefox. I use Firefox's extensions for my development work, and extensions like the Web Developer Toolbar are essential. Firefox has a Mac OS X version, so it was a simple matter of going to the Mozilla site and downloading Firefox.
Once again, this was a no-brainer. Thunderbird is a decent e-mail client, and partnered with the Lightning extension, can handle all the Outlook meeting requests thrown at it. Add in the LookOut extension, and it can even decypher those irritating proprietary winmail.dat files that Outlook uses to hide it's attachments (I still don't understand why Outlook can't be a normal mail client and send normal attachments).
Office Suite: NeoOffice/OpenOffice.org 3.0
OpenOffice.org 2.x for Mac needs X11 to run. I didn't feel like having to deal with X11, so I downloaded and installed NeoOffice, a native port of OpenOffice.org. Unfortunately NeoOffice is a little behind the stable OpenOffice.org releases, so when I heard that OpenOffice.org 3.0 was going to run natively on Mac OS X, I took the plunge and installed the alpha version.
OpenOffice.org 3.0 has worked well for me. I find it hangs a little from time to time (beta2 especially, RC1 seems to have improved), but overall it works well.
Mac OS X's terminal app just doesn't cut it. I downloaded iTerm and I haven't looked back - it's a simple but effective application. It has tabs for multiple sessions, it uses Ctrl rather than Apple, and the Home, End, PgUp and PgDn keys actually move the cursor.
Instant Messaging: Adium
At home I use Pidgin for my Instant Messaging client. It's feature-full, multi-protocol, and I've been using it for years. What better application to use for IM on my Mac than Pidgin's Mac cousin, Adium. While not an exact replica of Pidgin, Adium uses the same backend (libpurple) and mostly works the same.
Almost any Linux geek lives in a few IRC channels, and I am no different. Colloquy is a nice app that follows the Apple UI guidelines. I had to create my own theme (I like the white text on black background, which I couldn't find any themes for), but that was the only real problem I've had with Colloquy.
Music Player: Cog
As I said earlier, I found Audion after some searching, and it did the job. However, there were two things I wished for: Last.fm support and IM status message updating. The other day I decided to look through Adium's Xtras again and see if I could find anything.
The description of one of the scripts listed said, "This script shows your current Cog music." which got my interest. I did a search for Cog and to my delight found it to be open source, and it works much more like Amarok than Audion did. It also included Last.fm support (although you do need the Last.fm Mac OS X client) which was a second bonus for me.
Code Editor: KomodoEdit/OpenKomodo
Just like any other coder, I need an application that works with me when I write my code, rather than against me. I used a fantastic commercial application called Coda (which was how I found out about Audion), but unfortunately it's 30-day trial expired, and I had to look for another editor.
At the same time a friend told me about OpenKomodo, an open source version of KomodoIDE. It's not yet been renamed to OpenKomodo, so the application still calls itself KomodoEdit, but you can download it and use it. It's not as brilliant as Quanta+ but since about half my work is done on the serve in VI, I think I can live without Quanta+'s extra little bits. One of the big plusses for me with regards to KomodoEdit, howeverm is that the End and Home keys move the cursor. w00t!
Update: Today I opened KomodoIDE and it asked me if I wanted to start a 21 day trial. Evidently the pre-beta releases were free, but they've decided to keep KomodoIDE closed source. How disappointing. Back to KomodoEdit for me.