Here is some software that I like to install on any computer I am using:
copy-unc-path adds an item to the right-click menu of any file in Explorer that copies the text path to the clipboard. This is very useful in Revit when you have to enter a filename (DWG import, etc.) and saves lots of clicking up and down folders.
CCleaner has a startup configurator and is nice for cleaning the recycle bin, the registry, and uninstalling programs among other functions.
Clipboard Help+Spell is a text-based clipboard utility that keeps an accessible list of all of the things that you copy to the Windows clipboard. (Not to be confused with Revit's clipboard which is separate and has no history). It also has spell check and change case functions.
On-line Alarm Clock reminds you when to wake up or leave. I also use it to remind me when to move my car in Boston when it's at a meter. Just don't leave it unattended because it has a very annoying alarm sound.
Time Card Calculator & Timesheet Manager a great on-line calculator for converting work start and stop times to decimal hours.
Total Commander a super powerful alternative to Windows Explorer. Has a built-in FTP client, too. Search *.00??.r?? to find your Revit backups. (Not free per se but has just one nag screen.)
Air Duct Calculator -- who needs a cardboard ductulator?
ninite.com lets one install many different apps all at once. Great for setting up a new computer quickly. FastStone Image Viewer is a great app there for viewing and light editing of your images. Chrome, VLC, Winamp, CutePDF are all splendid apps.
Greenshot -- utility for screen captures. Grabbing a rectangular area of one's screen and putting the image in an email is an effective way of communication. (I did recommend Gadwin PrintScreen , but their newer versions don't work as well as their older versions.)
CleanMem is good for if you are running out of RAM when working with large RVTs. It seems to help performance. It can make the difference between painfully slow performance and being able to work if your RAM is limited.
AutoHotkey is a free utility for Windows. With it, you can automate almost anything by sending keystrokes and mouse clicks. Repetitive Revit tasks may also be automated.