Below are some decent freeware that I use (in alphabetical order). Note that
a) freeware is only free when it is for personal use.
b) when we want to install a few software, it is best that we install one first and restart the computer to see if there is any problem (e.g. system slows down significantly or the system hangs at shutdown). If there is problem, then we will know the software is not compatible with our computer (everyone's computer is different due to different software and different drivers). If there is problem, then uninstall the software, restart the computer and proceed to install another software. If there is no problem, then proceed directly to install another software.

7-Zip
7-zip.org
This file compression tool is able to open or create ZIP file. It is able to open RAR file.

AML Free Registry Cleaner
amltools.com
After Windows has been used for a few months, it tends to become more sluggish due to bigger registry size. I use AML Free Registry Cleaner to detect and delete obsolete entries in Windows registry.

AVG Anti-Virus Free Edition
free.avg.com
Every computer should have at least one antivirus. I use AVG besides a paid antivirus because a virus that cannot be detected by one may be detected by another. Anyway I turn off AVG's real time shield because I do not want it to clash with my paid antivirus during computer startup. I use it to scan the hard drive or partition where Windows load from occasionally. One problem with AVG Free is it cannot be updated for some computer. This problem can only be solved by reinstalling to the latest version from the website. Deleting or renaming updatecomps.bak or updatecomps.cfg, as suggested by some people, will not work.

ChartNexus
chartnexus.com
I use it to view candlestick chart of stock markets.

Everything
voidtools.com
I use it to get to a file on my hard drive quickly by typing in the file name or file format (*.mov for example). But it works only on hard drive that is formatted with NTFS.

Fast Duplicate File Finder
mindgems.com
This is not a software that I always use because once I clear away the duplicate files, I do not need to use it for a very long time. Fast Duplicate File Finder will compare files by exact file size and modified date even if the file name is different. I suggest to exclude folders that contain things that you want to keep 100% (such as important installers that have many files in them instead of just one setup file).

GIMP dialog box

GIMP
gimp.org
It is a picture editor that can even edit animated graphic interface format (GIF). It has features that are not available default Windows picture editing tool but available in paid picture editor, such as ruler and guide (e.g. center lines when cropping).

IrfanView
irfanview.com
It contains a picture explorer that can be set manually to view even unconventional picture format.

Spyware Terminator
spywareterminator.com
An anti-spyware provides extra security against spyware besides the antivirus. If we are sure that a file is part of spyware and it cannot be deleted by normal way, Spyware Terminator has a utility that can delete such file.

VLC Media Player
videolan.org
It can even play FLV and MOV format. The AVI movie format that does not work well on other media player works on VLC Media Player. It can also load SRT and SUB format subtitles.