There are a whole heap of tools I use in my job but some of them I could not live without. Here I have supplied links to the very best tools. Enjoy!

Sysinternals - Provided by Mark Russinovich, these tools provide great low level monitoring on the windows platform. Disk, registry, processes you name it Marks tools can do it.

Wireshark - Formerly Ethereal, is a java based utility for monitoring your network traffic. It provides support for a HUGE number of protocols and gives a real insight into what applications are doing in the background.

DotNetNuke - If your building a website or web application you cant go wrong with the the no 1 .net opensource product. It is extensible in so many ways and the market place around it provides additional functionality at low cost. This site is build on it!

CruiseControl - I dont see how any development team today can work without an automated build server. CruiseControl is simple yet very powerful.

ReSharper - Ok, its not free but the power it provides within visual studio is a must. Refactoring, deadcode, intellisence and othe tools make this product stand head and shoulders above the rest.

NUnit - Still a great unit testing tool for developers, falling behind recently with other test suits gaining tractions but it is free and simple to use.

Reflector - Lutz Roeder developer reflector for the .net 1.1 release and it has been invaluable. Allows you decompile any .net assembly and view in a range of languages.

NHibernate - Is an Object Relation mapper for the .net world. A port of the java implementation Hibernate, it allows you to work independantly from your datastore. No more ADO.NET.

AI-XAML - Mike Swansons amazing Adobe Illustrator to XAML converter. You can skipp past blend for the majority of design task's if you so desire. Check it out.

GhostDoc - Is a Visual Studio Add-in which allows developers to easily document their code by simply right-clicking and choosing "document this". Superb for time saving.

Kaxaml - Is an awesome XAML editor available free of charge when you dont have a copy of blend available. It provides decent intellisense and is great for prototyping.

LinqPad - LinqPad is a notepad style application that can be used to build and test your different linq queries. It covers LinqToObjects, LinqToSQL and LinqToXML.