Collection of favorite tools, programs, stuff. Most are free/open source, some are not.
Digital circuits
CircuitLogix - circuit builder/emulator
Logical Circuit - circuit builder/emulator
Simulo - circuit builder/emulator
Programming
Sim8085 - assembler/disassembler, machine code
Microsoft Small Basic - simple but powerful OOP, good for beginners
Eclipse - excellent programming environment for i.e. Java, C++, Android and so on
ZGameEditor - 3D videogame editor
3D Rad - powerful WYSIWYG 3D videogame editor, easy for kids
Physics toys
OECake - 2D fluids physics editor, simulator, easy for kids
Powdertoy - 2D particle/physics editor, simulator, more advanced
Editors
Axialis icon Workshop - icon editor
MakeHuman - 3D character editor
FaceGen - 3D face editor
Structure Synth - creates 3D structures with simple script
Educational
Yenka - Excellent simulators for kids, teachers. Mathematics, electronics, chemistry, physics
Realistic simulators
Orbiter - Realistic space flight simulator, many add-ons
ARMA2 - Free. Massive realistic military combat simulator in a vast landscape
FSX add ons/tools
ASE Active Sky Evolution - FSX weather, excellent
NL2000 - FSX Netherlands in high resolution
FS Water Configurator: Enhances the look of water.
Venetubo: Upload your ini file and download a tweaked one. Excellent results.
3D tweak
ENBSeries - Tweaks DirectX, major improvement in many 3D games like FSX, GTA, ARMA
Astronomy
Celestia - astronomy - in space
Stellarium - astronomy - from earth
Space Engine - incredible universe simulator, fav
Fractals
Terragen - fractal landscape editor, render 3D. Free for non-commercial use.
Mandelbulb3D - 3D fractal explorer, animator, editor. Favorite.
GNU XaoS - fast Mandelbrot fractal. Fav.
Mandelbulber - another 3D fractal explorer
Apophysis 7x - 3D fractal flames
Chaoscope - 3D fractals flames, animations, render 3D. Fav.
ChaosPro - 3D fractals
Fr0st - 2D fractal flame creator
Fractal Harmonies - Old but impressive. Mandelbrot/Julia to music melodies, including orbiter
Fractice - Mandelbrot explorer
Fractron 9000 - 2d fractal flame art Sierpinski type
Incendia - 3D multi processor fractal engine, renderer
System tools
CPUID - system info HW
PowerGUI - system info Windows
SIW - System information Windows, installation keys/serials and password recovery
Speccy - sys info by Piriform
Speedfan - CPU cooler fans reader/setter
Fiddler2 - web debugger proxy, traffic logger
UltraEdit - file editor, $
Universal File Viewer - preview/open every file via right mouse button
MagicDisc - virtual DVD drives. Mount ISO
powerISO - virtual DVD drive. Mount ISO, DAA
inSSIDer - WIFI troubleshooter
NetStumbler - WIFI troubleshooter
Wireshark - network traffic logger/capture/analyzer, fav
MyDefrag - good disk defragmenter, optimizer
Recuva - deleted files recovery
CCleaner - cleans temp files, caches. Be careful not to wipe shortcuts.
TFC - Temporary file cleaner. Very robust. Close all programs before running, may require restart
Process Explorer - Microsoft replacement of the default task manager, far more options, views
Recording
CamStudio - screen recorder
ManyCam - webcam broadcast, emulator, real time FX, subtitles, logos
Free Sound recorder - just record 'what you hear' on the desktop
K-lite codec pack - windows media player classic + many codecs, filters
Video effects, electronics
vvvv - real time video FX, art, animation creator, editor, player
pd (Pure Data) - real time graphical programming environment for audio, video
Arduino - open-source electronics prototyping platform
Fritzing - electronics design
Processing - create images, animations, and interactions
WinTTX - Teletext Ceefax editor
3D modelling
Google Sketchup - simple but powerful 3D modeller
Wings3D - 3D modeller
Misc
Framsticks - older software but still very impressive. Artificial 3D life with 'DNA', evolution