As most developers, I've got my chest of tools that I rely on daily to be productive.  Here's a list of the apps that I don't think I could live without:

Internet Explorer This is needed mainly for webmail and SharePoint.
Maxthon I've use this for my SSL VPN. It is a MDI interface ontop of IE but has a big memory footprint so I don't use it for everything IE.
FireFox I use this for everything else that doesn't require IE. Also, when I need to test a website from multiple user contexts, it's nice to have a two browsers with different cache settings.
VIM I've also got a VERY fast, VERY lightweight command line VI program that I run when on the command shell in windows. I'm not sure where to get it as it's timestamped 1994.
Microsoft Visual Studio  
ActivePerl Perl for Win32.
SOAP::Lite My SOAP debugging language of choice.
XMLSpy If you interact with XML, this is a must-have.
Eclipse Open Source Java Dev Environment
TCPTrace Great TCP Proxy for debugging HTTP based web services.
PuTTY PuTTY is a free implementation of Telnet and SSH for Win32 and Unix platforms, along with an xterm terminal emulator
Snippet Compiler Compiles and runs single or multiple C#, VB.NET and ASP.NET snippets.
Reflector for .NET Reflector is a class browser for .NET components.
Everthing from pslist, pskill, procexp, regmon, filemon, ... Too many to mention.
Skype Free Internet Telephony
w.bloggar Windows based weblog posting app.
Notepad2 Great Notepad.exe replacement
Microsoft Virtual PC OS Virtualization

Please let me know if there is anything that you use! -Joe