IT System Inventory and Monitoring
Spiceworks 3.0 - Windows based. Smooth graphics, easy to use, great support.
Open-Audit - Windows or Linux based. Great because you can easily customize it, and it can do some things Spiceworks cannot...namely adding custom software entries based upon your own criteria.
Remote Desktop/Control
Royal TS - Support for RDP only, but is VERY fast and easy to use. This is very well put together.
Terminals - Features support for RDP, VNC, ICA, Telnet, SSH, VMRC and RAS! This one is gaining favor with me :)
Logmein - Remote control any PC over the Internet without remote prompting
Crossloop - Easily remote control a non-technical user's computer by installing a single app and having them share their remote access code.
TCP/IP Tools
SoftPerfect Network Scanner - Scan a subnet, IP range, or a single IP for details about ping stats, open shares, printers, open ports, SNMP info, etc. Also features an easy right-click ability to connect to the PC via HTTP, telnet or Windows Explorer.
WireShark - Packet sniffer/monitor/network protocol analyzer
Putty - secure telnet to your network devices. Customizable and small.
IReasoning MIB Browser - Use this to browse the various SNMP properties of your network devices. Great if you are trying to figure out that OID for monitoring purposes.
Random network tools
Net Tools - a HUGE collection of a variety of network tools...whois, finger, encrypt files, password generator...the list goes on, and on, and on... :)
Productivity
Synergy - Seamlessly work on two computers at once with one keyboard and mouse.
File transfer/FTP
Filezilla - Constantly improved FTP program, easy to use, more features than some paid programs. Features a server program as well.
WinSCP - Also easy to use, WinSCP features a couple additional secure-transfer options that FileZilla does not.
Active Directory/Windows Networking
AD Explorer - Quickly browse and view or edit your AD LDAP structure.
ADRestore - Undelete tombstoned AD objects.
ShareEnum - Quickly export file/folder permissions with little work on your part.
SpecOps GPUpdate - A plugin to your ADUC, you can right-click a single or set of multiple computers to restart, logoff, shutdown, or force the computer to re-pull applicaple group policies.
Netwrix Inactive Users Tracker - List users that haven't logged on for x days, email a report.
Text editor
PSPad - Syntax highlighting, endless features, HTML cleanup and much more.
Other tools
Microsoft Scriptomatic 2.0 - Get WMI information from remote or local computers...a must have if you write VBScript, JScript, or Perl based system administration scripts.
Universal Extractor - Extract contents from just about any kind of file, setup executables, Zip/Rar (and others)...very handy!
AutoIT - VERY quickly and easily write automation scripts, small programs and routines. Lots of built-in macros.
Spotlight on Windows - View a dashboard of system-specific info, complete with red/yellow/green highlights to help you drill down to problem areas quickly.