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Handy software and utilities
Internet Radio Streams
EditPad 3.2.1 (Excellent Notepad replacement)
Key Tweak (Keyboard remapper) (XP, 7?))

Power & Kernel Toys (Microsoft)
TweakUI 2.10 (WinXP)
SendTo (Win98 / XP / Win7)
Command Prompt (XP)
SyncToy (backup utility) (XP) (Will prompt you to download and install the .NET Framework from Microsoft)
TaskSwitcher (Alt-Tab replacement) (XP)
Process Explorer (Beefed up Task Manager shows everything running) (XP / Win 7)

Secure Internet
Hosts file replacement This file can be used to block annoying ad sites and pop-up malware such as "Antivirus2009". Download and replace "c:\windows\system32\drivers\etc\hosts" (no file extension). Here is a link to a site that does the same.)

Firefox Extensions
Since these change constantly and may not be compatible with the current version of Firefox I'll just leave the home page links.
WebOfTrust (greatly improve safe surfing) - Homepage
Undo Closed Tabs Button - Homepage
Webmail Notifier - Homepage
MinimizeToTray (for Thunderbird) - Homepage

See my recommendations page for more about these indispensible pieces of software)

OperaOpera 12.18 - the last version before it became a Chrome imitation. Vivaldi may become the preferred fork of this excellent browser.
64 bit version
Get FirefoxFirefox 52 - the last version for XP. Visit the homepage for the current version.
Get ThunderbirdThunderbird 2.0 - My preferred version of this stand-alone email client. Greatly changed with version 3+ (tabs).
Spybot S&D Crap Cleaner 4.07 - Homepage
MalwareBytes MalwareBytes' Anti-Malware 1.75 - Homepage for the updated version (not sure I like the new interface)
Spybot S&D Spybot Search & Destroy 1.62 - Homepage
Winamp Most excellent media player - Homepage
Vacuum Tube skin for Winamp

Older XP Utilities
Key Tweak (Keyboard remapper) (XP))
Royale/RoyaleNoir (Themes for XP from Microsoft). Royale(default theme for Media Center Edition) - to tone down the neon bright default of XP and Royale Noir (with wallpaper) - to give it a Vista look without having to install Vista. Changing the default theme can also alert you to malicious popups.)
Classic Win98/2k Utilities
TweakUI 1.33 (Win2K)
WinTop (Win98)
KeyRemap (Win98)
DosHere (command prompt) (Win98)