I use many open source/freeware products in my computer. Most of them might be used by you! However, I wish to list out them here, so that, you can also get benefited by a nice product, and I may also get benefited if you know a better product than in my listing.
Thanks.
Here is the list of products for your kind information. I will update this, if I know some more in the future too.

1. Software development - of course HMG. Any doubt about this?!
2. Code editor - Crimson Editor (Thanks Esgici for introducing Crimson to me which is along with syntax highlighting for xBase) or Notepad++
3. File/Code comparison software - WinMerge
4. Mind Mapping Software - FreeMind
5. Image Viewer/Slideshow/Thumbnail creator - IrfanView
6. EMail Notification Software - SimpleCheck
7. Compression/Decompression Software - 7-Zip (For handling .zip, .rar, .gz, .bz etc.,)
8. Audio Manipulation - Audacity
9. Paint for kids - TuxPaint
10. Universe Explorer (Simulation) - Celestia
11. PDF Creation through print - PDF Creator
12. Windows Process/Start up information - What's Running
13. Educational Kit for kids - GCompris
14. Simple Animation Software for kids - Pivot Stick Animator
15. Enhanced Explorer - ExplorerXP
16. Anti-Virus - MoonSecure (Along with ClamAV)
17. Mobile Media Converter - Mobile Media Converter
18. BitTorrent Client - uTorrent
19. Registry Tracker - RegRunner
20. LAN Management - Look@LAN
21. Live Dictionary - Star Dictionary (Available for many languages also)
22. Virtual Network Computing (VNC) - TightVNC
23. Screen Capture/Animation - CamStudio
24. Small MP3 Player - XMPlay
25. Unit Converter - Das Unit Converter (Believe it! More than 2000 units in 80 categories!)
26. Quiz Software - Qedoc Quiz Player
27. Desktop Calendar, Event Log, Reminder - RainLendar
28. Panorama Photo Stitcher - Hugin
29. KDE utilities for Windows - KDE4Win
30. Digital Simulator (For Electronics Students) - Digital Simulator
31. Project Management - OpenProj
32. Clipboard Manager - Ditto (Very useful stuff)
33. Movie Subtitle Editor - Subtitle Edit
34. Nutrient DataBase - Nutrient Data
35. Personal Information System, Notes etc., - Total Text Container
36. PDF Viewer - SumatraPDF (Believe me, I don't use Adobe PDF viewer!)
37. Merge, Split PDF Doc - PDFTKBuilder
38. File Eraser - Eraser
39. One Line Calculator - SpeedCrunch
40. Multi-desktop for Windows - Windows Pager
41. Windows Hider - ClickyGone
42. Image Manipulation - GIMP (With Animation Package)
43. Office Suite - OpenOffice.org (Along with Sun PDF Importer Extension)
44. E Mail Client - Thunderbird
45. Internet Browsing Software - Firefox (With some useful extensions like DownloadThemAll)
46. 3D Stereo Photo Maker - Stereo Photo Maker (Also Video)
47. Instant Messenger - Miranda (GTalk, Yahoo!, AIM,... all in one place)
48. Comic Reader - CDispEx
49. Help File Creator - Shalom Help Maker (For old fashioned creation of .hlp files - to integrate with HMG help ids!)
50. Help File Creator (CHM) - HelpMaker
51. Setup File Creator - InnoSetup
52. Personal Firewall - Jetico Personal Firewall
53. Media Converter - MediaCoder
54. Media Player - VLC Media Player
55. Medial Player - Media Player Classic
56. Sudoku - Simple Sudoku
57. Planetarium - Stellarium
58. RDBMS - MySQL
59. Vector Based Drawing - Inkscape
60. 3D Animation - Blender
This list will be categorized soon. Thanks for your patience!