TextMate Ñ The Missing Editor for Mac OS X

Mac OS X Hints - A community-built collection of OS X hints

WineHQ - Run Windows applications on Linux, BSD, Solaris and Mac OS X

Open Source Software for running Windows applications on other operating systems.

TrueCrypt - Free Open-Source On-The-Fly Disk Encryption Software for Windows 7/Vista/XP, Mac OS X and Linux

TrueCrypt is free open-source disk encryption software for Windows 7/Vista/XP, Mac OS X and Linux. In case an attacker forces you to reveal the password, TrueCrypt provides plausible deniability. In contrast to file encryption, data encryption performed by TrueCrypt is real-time (on-the-fly), automatic, transparent, and does not involve temporary unencrypted files.

apache friends - very easy apache, mysql, php and perl installation without hassles

XAMPP is an easy to install Apache Distribution for Linux, Windows, Mac OS X, and Solaris. The package includes the Apache web server, MySQL, SQLite, PHP, Perl, a FTP-Server and phpMyAdmin.