I just can’t live without these tools! (literally speaking of course!) Arrange randomly ( I just can’t decide!)
#1 MyNotesCenter 
Price :24.95 Its not free yet the most useful tools for my daily work. All notes, scripts,license info, passwords which are used most of the time for my works arranged in tree like organization. Automatically loaded once my PC is started and saved before logging off.
For those who are budget concious may consider this TreePad
VIM
Old fashion guy like me still prefer Vi! 
Ethereal http://ethereal.com/
Sniff sniff! Best network troubleshooting tools around (free punya laaa) ? 
Irfanview http://www.irfanview.com/
A light but powerful image viewer! Being a support engineer (a lot of time) I have to send screen captures of problems to Product Line, view image files (I know whats in your head!) and Irfanview always been the handy tool. 
Putty http://www.putty.nl/
I tot I tee a putty cat! I don’t think it needs more introduction.
PuTTY is a free implementation of Telnet and SSH for Win32 and Unix platforms, along with an xterm terminal emulator. It is written and maintained primarily by Simon Tatham.
Subnet Calculator
There are a few but Solarwinds’ is my favourite. Its free too! 
Network/Unplugged http://www.mobiliti.com/netUP.html 
Its not free and installed in all notebook in my organization. What its all about?
Network/Unplugged provides a comprehensive and automatic synchronization and backup solution for today’s work force. Users can operate in a Virtual Network environment when detached from the network, making Network/Unplugged’s synchronization solution extremely easy to use. Network/Unplugged’s proprietary technologies enable fast synchronization and backup (including outlook pst files) ensuring that only the portions of files that have changed are transmitted during synchronization and backup including outlook. Network/Unplugged provides the enterprise a cost effective software solution that is lightweight and scalable with deployment and administration support. Features in Network/Unplugged 5.0 * Open File Backup for PST Files * Offline Files and Folders * Incremental File Backup and synchronization * Adaptive file level differencing * Synchronization over internet
http://www.realvnc.com/
I haven’t use this for ages (it was my favourite maybe 10 years ago!) but recently there was a request for a free solution for a remote PC control from a customer , so here it is. 
Qcheck http://www.ixiacom.com/solutions/display?skey=qcheck
Qcheck – Network Performance Measurement What’s good-looking, powerful, improves your quality of life, and is FREE?
Qcheck, the network troubleshooting utility from Ixia, slices, dices, and checks network response time, throughput, and streaming performance. It even runs anywhere-to-anywhere traceroute!
