I really enjoy open source software. It is somewhat lost on me why most enterprises are so heavily dependant on proprietary software when the open source equivelants are better and/or more widely used (Apache/nginx, Linux, bash, git, etc.), or the project the software was purchased for certainly doesn't merit the cost or need the support (eg: an Oracle instance for a four column single table test database, really?).
All that goes to say though that I really like open source software. I try my best almost to use it simply on principle, overlooking the shortcomings of some of the projects just because the code is open and available. With that, here are some of my favorite projects right now (not necessarily in order or awesomeness)
Linux: I'm a fan of Debian (yeah yeah yeah, I don't want to hear about the political war going on around this one).
Arch Linux: Severed limb style bleeding edge Linux distribution. Requires a medium to high level of Linux experience to even get it installed unless you're looking for a really great Saturday afternoon project (because what IT guy wakes up before 11:00 on a Saturday anyways).
Git: Code Versioning System. Not easy for mere mortals at first, as someone once said. It sure is amazingly powerful.
MediaWiki: Standard wiki but it's really great for blogging, team documentation, project documentation tracking, etc. Wikipedia runs on this.
PHP Server side script language. Fantastic for building websites. Also really amazing replacement for Perl for server side scripting in my opinion. Supports object oriented programming and is under constant development. I really can't say enough good things about this project.
MariaDB: Forked from the MySql project by its founder, Monty Widenius when the open source grimm reaper...er...Oracle/Sun acquired it in 2008 (no, I'm not opinionated at all).
Apache / NGINX: Two really solid web servers that currently occupy the top two slots on the most widely used web servers in the world (yay for open source!)
EJabberd: Pretty popular XMPP server software with a good developement speed. Built with erlang.
Arch Linux: Hey look! I used this one twice! ... Severed limb style bleeding edge Linux distribution. Requires a medium to high level of Linux experience to even get it installed unless you're looking for a really great Saturday afternoon project (because what IT guy wakes up before 11:00 on a Saturday anyways).
XFCE: Desktop interface for Linux.
Pidgin: Instant messaging client
LibreOffice: Document authoring (MS Word, Excel, PowerPoint, etc)
GNU Core Utils: chmod, chown, ls, cut, tail, wc, su, w, etc. Full list of commands provided can be found here. Basically, if you use Linux command line ever, these are the majority of the tools you use most frequently. Despite all the politics surrounding Mr. Stallman and his amazing beard, the GNU project has done a great job with the core utils, bin utils, diff utils, file utils, and everything else they've done.
Blender: 3D modeling (not for the faint of heart)
Inkscape: Vector imaging. Good replacement for Adobe Illustrator
KeePassX: Encrypted password database for Linux. Windows equivelant is KeePass.
Gimp: Great replacement for Photoshop, especially given that most people purchase Photoshop [Elements] and use it to crop and autocolor images. I can't speak highly enough of this software. It may take a bit to learn, but it is tremendously powerful. Here is a great list of images that people have made using Gimp.