ya i know we've done this one before lol
anyway, thread title is self-explanatory...
just please link to whatever it is you refer to, and a brief description, and please try to avoid posting anything already posted...
edit - also, please avoid any debates of one thing being better than something else (unless it is asked), just please post things you have found useful

ill begin:
Microsoft Security Essentials provides real-time protection for your home PC that guards against viruses, spyware, and other malicious software.
Malwarebytes has a variety of tools that can identify and remove malicious software from your computer; and if your computer becomes infected, Malwarebytes can provide the needed assistance to remove the infection and restore the machine back to optimum performance.
CCleaner is the number-one tool for cleaning your Windows PC. It protects your privacy online and makes your computer faster and more secure. Easy to use and a small, fast download.
Glary Utilities is the #1 free, powerful and all-in-one utility in the world market! It offers numerous powerful and easy-to-use system tools and utilities to fix, speed up, maintain and protect your PC.
TweakNow Registry Cleaner is an excellent freeware utility makes the potentially daunting prospect of cleaning your Windows Registry safe and simple.
ToniArts Easy Cleaner is a small program which searches the Windows registry for entries that are pointing nowhere. EasyCleaner also lets you delete all kinds of unnecessary files such as temps and backups. You can search for duplicate files and you can view some interesting info about your disk space usage! You are also able to manage startup programs, invalid shortcuts and add/remove software list.
Auslogics Disk Defrag is a compact and fast defragmenter that supports both FAT 16/32, and NTFS file systems. It is supplied with advanced optimization techniques, which will remedy your system sluggishness and crashes caused by files fragmentation.
FileHippo Update Checker will scan your computer for installed software, check the versions and then send this information to FileHippo.com to see if there are any newer releases. These are then neatly displayed in your browser for you to download. Please note that not all programs are supported.
Process Explorer shows you information about which handles and DLLs processes have opened or loaded. Ever wondered which program has a particular file or directory open? Now you can find out.
Revo Uninstaller is a powerfull uninstaller, which you can use to uninstall programs and scan after uninstall for leftover registry keys, files, folders, and registry keys that are usually left over on your computer.
The Weather Channel Desktop provides instant severe weather alerts, extended forecasts, interactive maps, radar and much more directly from The Weather Channel.
Audacity is a free, open source software for recording, editing, slicing, and mixing audio with a whole lot more options.
Belarc Advisor builds a detailed profile of your installed software and hardware, missing Microsoft hotfixes, anti-virus status, CIS (Center for Internet Security) benchmarks, and displays the results in your Web browser.