Here is a list of some of my most used tools from my software tool kit.
WinDirStat - A quick way to tell what files are filling up your hard drive. A must have for cleaning up full hard drives
SpinRite Boot CD - Hard Disk recovery tool
CCLeaner - Tool for cleaning up temp files on your computer
GParted Live - Resize partitions
Knoppix 5.11 - Live linux boot CD with a ton of tools.
UBCD for Windows - Run windows xp from a CD. Has a ton of tools
Offline NT Password & Registry Editor - Boot CD that can change local account passwords
Kon-Boot - Log into any local or domain account on a pc without a password. Very cool tool.
Acronis True Image with Universal Restore - Disk Imaging tool
HDClone Pro - Hard Drive cloning tool
Malwarebytes - Anti-Virus/Malware
VIPRE Rescue - Anti-Virus/Malware
JKDefrag-3.36/MyDefrag-4.0 - Hard Drive Defrag program
Sysinternals - An sysadmin's best friend
Putty - Telnet/SSH Client
Superscan 3.0 - fast little port scanner. A quick way to tell whats on the network.
Microsoft Network Monitor 3.4 - Microsofts updated packet sniffer. Much improved!
NEWT - A very fast network inventory tool.
DBAN - Securely wipes hard drives to DOD standards
Blue Screen View - Scans mini dumps to help translate blue screen of death crashes.
Virtual Box - Run Virtual Machines. I use it to run everything from Windows 95 to Windows 7, and Ubuntu. What's best is it has 3D support for windows and Linux so it's functional and pretty.