A list of applications that has passed the Andy meter as
being top quality, grade A computing fun. I found most from another
great OSDN website, Freshmeat:

Databases
MySQL - A very popular, very fast (for SELECTs) database
PostgreSQL - A very complete ANSI-SQL dbms unlike MySQL

Operating Systems
FreeBSD - Very stable and very cool OS

PHP
Gallery - PHP photo management application
phpMyAdmin - MySQL PHP Frontend to make managing your databases simple and fun
SiteBar - A very rich bookmark application with the ability to share bookmarks with other users
Squirrel Mail - A clean, simple to use webmail frontend

Programming
ActivePerl - Free, Win32 compilation of Perl
ADOdb PHP Library - Makes using any data source transparent to your php application
PHP - Hypertext Preprocessor scripting language at a very good price
FCKEditor - An Internet Explorer in-browser HTML WYSIWYG editor with support for ASP, ASP.NET, PHP, Coldfusion.

Networking/Mail/WWW
Apache HTTPD - Widely used open source web server
CourierIMAP - IMAP interface to maildir stored mail systems
CUPS - Common UNIX Printing System with support for SAMBA
SAMBA - SMB/CIFS file service

Utilities
Putty - The best Windows console SSH client around, hands down
SmartFTP - A very easy to use graphical FTP client
WinMerge - An excellent replacement for old school WinDiff
WinSCP - An SFTP/SCP graphical interface

Portable/Thumb Drive Apps
TrueCrypt - Provides on-the-fly encryption via virtual disks on multiple platforms
PasswordSafe - E-wall password manager