I use a couple of programs that I probably couldn't live without... most of them are portable (or can just be run without needing to be installed), and all of them are free. Combined they weigh in at under 20mb - a very small price to pay for the functionality.

I've listed them in alphabetical order, and will provide a brief description of what they do and why I use them, but it's up to you to read more about them and try them out:

allSnap - enabled "window snapping", like WinAmp and Steam do... just helps with positioning windows on the screen

DisplayFusion - allows you to set different wallpapers on different monitors and provides hotkeys for moving windows between monitors, when you have more than 1 monitor (yes, I know, you could just use UltraMon, but it's not free - though it does offer more features)

Hamachi - allows you to "network" computers over the internet, works through all kinds of network setups and firewalls

KatMouse - by default Windows scrolls the "active" part of the "active" window when you use your mouse scroll whell, this program scrolls whatever's under your mouse

Launchy - basically a "Run" menu on steroids, you tell it the directories and file types to monitor and it makes launching files or apps incredibly quick and easy

PowerMenu - provides extra menu items when right-clicking on items in the taskbar, allowing you to adjust a program's transparency, minimize it to the system tray, make it "always on top" and adjust it's priority

ProcessTamer - monitors running programs and lowers the priority of any using too much CPU usage so your PC doesn't slow down or freeze (only started using this today, but it looks like a keeper)

StrokeIt - gives Windows "mouse gestures" (you hold down your right mouse button and drawing shapes/letters) to execute commands, maximize/minimize windows etc (DEFINITELY can't live without this one)

Taskbarshuffle - allows you to reposition items on your taskbar or in your system tray by dragging them, and close them by clicking your middle mouse button (like FireFox tabs)

UltraVNC - allows you remotely access/control your computers (I use this with Hamachi to access my home pc from work and visa versa)

Unlocker - when ever you get a "file in use" error (eg: when trying to move or delete a file) this program will tell you what's using the file and allow you to close that program or unlock the file

I hope you find these tools useful :)