By all means, this is far from a complete list of my .NET development toolbelt. It is just a select few of effective lightweight utilities that I commonly use. Often said, although there is often a better tool for the job, sometimes the best tool for the job is the one you know. Please feel free to share some of your own favorites.
Productivity & Desktop
dnGREP
Developing and working within Unix (I often date myself by typing UNIX) and Linux environments, I've learned to really appreciate some of the great shells and commandline utilities. One of my favorite utilities was grep, which is a text searching utility. dnGrep is a Windows implementation of this utility.
http://code.google.com/p/dngrep/
Cygwin Bash Shell
Tee'ing off of the previously stated usefulness of the Unix command shells, Cygwin is a port of many of the more popular Unix tools and provides commandline interaction of these utilities.
http://www.cygwin.com/
Focus Booster
This is just one of a dozen-dozen utilities assisting with the Pomodoro Technique. This is just one that I like because it's simple, clean, and small. That's kind of a theme of mine when it comes to maintaining my development toolbelt.
http://www.focusboosterapp.com/
If you want to learn more about the Pomodoro Technique have a look here: http://www.pomodorotechnique.com/ Obviously, this isn't the end-all be-all of productivity methods. There may another one that's a better fit for you.
Input Director
Multiple computers, multiple monitors, one keyboard, one mouse. There's nothing that keeps my desk cleaned up and my hands in one place better than this productivity tool.
http://www.inputdirector.com/
notepad
Maybe overstating the obvious, but notepad is one of my more useful tools as well. Often I'm on a system without a good toolset and need to remove any textual formatting from any text that's block copied. While my development machines have various utilities for pasting cleaned up text, that system on the other end of the network probably will not.
ImgBurn
As often, simple is always better. I've created so many less coasters over the years with this CD/DVD burning/copying utility than with any other tool.
http://www.imgburn.com/
FormatFactory
This is a just a great media converter. It's been very effective for me and is one of the few that doesn't bog down my workstation.
http://www.pcfreetime.com/
Graphics & Images
Paint.NET
This is probably my alltime favorite graphic editor. It is such a capable image editor and has a large following with which to collaborate.
http://www.getpaint.net/
Greenshot
Greenshot is a speedy lightweight screenshot tool. I'm hesitant to keep anything running in my windows taskbar that I don't need at all times and is going to use up a lot of memory. These kind of utilities are notorious for sapping away more memory than they need while waiting for you to use them. Greenshot is not one of them and has a home in my taskbar.
http://getgreenshot.org/
Wordpad
Did I hear a laugh? Yes, as funny as it sounds. Often I need to get full or partial screen shots on systems that don't have much of anything in terms of productivity tools installed on them. I sometimes need to gather a few simple screen shots and to move on. I've found that it's quicker, and tidy, to just drop a bunch of images in a Wordpad document and then wrap things up elsewhere.
Reporting
RSScripter
Although, it can be done from a manual sense, I often need to grab a load of SSRS reports from a server to either be moved or worked on. This little utility is great for short work of these operations.
http://www.sqldbatips.com/showarticle.asp?ID=62
Report Builder
I often say that developers are lazy. They only want to type something once, automate things, and build things that, in turn, build things for them. Every once in awhile a dynamically generated report doesn't fit the bill and a one-up report is needed. I don't want to take up time with a project, I just want to get the information out there into a report format as quickly as possible. Better yet, if all of the work is done in a stored procedure and available to the report definition, I only need to worry about the layout, groupings, etc. of the report.
http://www.microsoft.com/en-us/download/details.aspx?id=6116
Internals
Dependency Walker
This great utility by Steve Walker examines Windows assemblies and provides realtime interactive diagrams of any activity, errors, and interactions between the different modules running on your system. While it provides extremely detailed useful information, it is surprisingly intuitive for the uninitiated.
http://www.dependencywalker.com/
Sysinternals
Ever since I first purchased the Winternals suite of admin tools years ago, I've been a satisfied customer. This suite of small and effective utilities are use for monitoring and managing Windows systems. Microsoft later obtained Winternals and still has the majority of the tools available for free. I often create scripts or shell commands to the various utilities. Process Explorer, TCPView, PsService, PsList, PsKill, PsInfo, PsExec are the utilities I make use of on a regular basis.
http://technet.microsoft.com/en-us/sysinternals/bb842062
Development/Debug
Firebug
This is a Firefox plugin that provides a more detailed view of HTML, CSS, and JavaScript from the client.
http://getfirebug.com/
Glimpse
This is an ASP.NET diagnostics tool that captures performance characteristics for further debugging or troubleshooting. It has the ability to gather an amazing amount of detail fairly easly.
http://getglimpse.com/
JustDecompile
Reflector used to be my go-to tool for .NET code decompilation. This has been an incredible useful tool. Not long after Reflector became a premium product, JustDecompile from Telerik came on the scene.
http://www.telerik.com/products/decompiler.aspx
FXCop
FXCop is tool that helps maintain that certain rules pertaining to security, naming conventions, proper memory usage, and design are met within .NET code.
http://msdn.microsoft.com/en-us/library/bb429476(v=vs.80).aspx