Last night at the BAND (www.bangordevelopers.com) meeting we all did 10 minute presentations on the tool(s) we love as developers.
On my list were: Total Commander, LinqPad, Microsoft Office (codegen with Excel rules 
), VMWare & VirtualPC, Live Mesh. Also on the list should have been Reflector.
Total Commander
Great tool for FTP-ing files and comparing a local folder hierarchy to the hierarchy on the ftp-server. Also much more reliable in FTP-ing large amounts of files than Windows Explorer.
See: www.ghisler.com
LinqPad
A must have for people using LINQ to SQL and Entity Framwork. Helps a lot with figuring out what the exact SQL statement is that get generated from your LINQ statement.
See: www.linqpad.net
VMWare Converter, VMWare Workstation and VirtualPC
These tools are invaluable in creating clean testing environments and separating multiple development environments on a single machine. VMWare converter allows you to grab a physical harddrive and convert it into a virtual machine. Very useful for Windows 7 migration scenarios!
See: www.vmware.com, www.microsoft.com/virtualpc
Live Mesh
Is only in beta, but already an invaluable tool for remote desktop connections across firewalls and synchronizing files across (virtual) machines.
See: www.mesh.com
Reflector
.NET Reflector is a tool any serious .NET developer cannot do without. View sources of any .NET library you use in order to track internal workings. Love it!
See: http://www.red-gate.com/products/reflector/