Xfce is a lightweight desktop environment for various *NIX systems. Designed for productivity, it loads and executes applications fast, while conserving system resources.
wmii is a small, dynamic window manager for X11. It is scriptable, has a 9p filesystem interface and supports classic and tiling (acme-like) window management. It aims to maintain a small and clean (read hackable and beautiful) codebase.
VirtualBox can seamlessly integrate Windows into Linux. Proprietary version is free for personal/educational use, and the GPL'd version has almost all the same features (no RDP, no USB, no iSCSI).