This is a very rough draft of some of the tools I use in no real order. The links are too the parent home page or a download page.
The end all awesome place to find open source projects and downloads:
Source Forge
Windows configuration:
nLite - nLite is a tool for pre-installation Windows configuration and component removal at your choice. Optional bootable image ready for burning on media or testing in virtual machines.
CCleaner - CCleaner is a freeware system optimization, privacy and cleaning tool.
SIW - SIW is an advanced System Information for Windows tool that gathers detailed information about your system properties and settings and displays it in an extremely comprehensible manner.
Unlocker - How often have you tried to delete or rename a file or folder and got
"Cannot delete xxx: It is being used by another person or program."? Unlocker is a tool which will help you overcoming this scandalous Windows bug.
Text & code:
Notepad++
Image editing
Paint.net - Stop pirating Photoshop! Paint.net adds what I would consider basic image editing to what basically looks like MS Paint.
Arrchiever:
7-Zip - An anything archive related must.
Network and Security
AngryIP Scanner - A fast and easy network host scanner. Basic info and, as I said, fast.
Metasploit
Cain (& Able) - The network scanner/sniffer/cracker, password recovery tool for windows. Constently updating and very well made.
nmap, zenmap, umit - nmap is a very comprehensive port and host scanner. Maybe not the fastest, but the best for information. Zenmap is a GUI front end for nmap. umit is much the same as zenmap, but I've grown to like it.
WinSCP - WinSCP is an open source free SFTP client and FTP client for Windows. Legacy SCP protocol is also supported. Its main function is safe copying of files between a local and a remote computer.
PuTTY - PuTTY is a free implementation of Telnet and SSH for Windows and Unix platforms, along with an
xterm terminal emulator.Wireshark - Wireshark is a network protocol analyzer for Unix and Windows.
Win32whois - find out more about a domain with out all those annoying pop-ups web based WhoIs services have.
SoftPerfectNetScanner - Multi-threaded IP, SNMP and NetBIOS scanner for Windows with a modern interface and several advanced features.
Mulimedia:
VLC - The everything player.
Boxee - Nice iTunes/MediaCenter replacement. Still young, but lots of promise.
Other:
Maltego - Internet data miner.
PowerToys - PowerToys add fun and functionality to the Windows experience. By Microsoft.
This is just a start. I encourage you to comment and help me improve this list.
