The table below lists various tools that can be of use when troubleshooting certain aspects of development. To save myself some typing, I consider everything I've come across at SysInternals (feed) to be useful, but in particular have used Process Explorer, FileMon, RegMon, TcpView and DebugView on many occasions.

Tool Author Area Comments
ASP.NET Dev Helper Tool Nikhil Kothari ASP.NET development IE plug-in to help with various ASP.NET issues (trace output, view state, etc.)
"ClickOnce" log n/a "ClickOnce" deployment Longhorn only: in %USERPROFILE%\Local Settings\My Applications\System.Deploymentframework.log
Code download log viewer Microsoft IE code download See also this KB entry
Component Checker Microsoft MDAC Useful for those "well it works on my machine" moments
ComTrace Jonas Blunk COM Intercepts and logs COM interface method calls, and various COM API calls
Detours Microsoft Research Win32 Allows arbitrary interception of Win32 functions
Developer Playground Jonas Blunk Win32 Intercepts and logs any DLL call (including, of course, Win32 API ones)
Fiddler Bayden Systems HTTP Allows HTTP traffic to be monitored and modified in flight between browser and web server
FusLogVw Microsoft .NET assembly loading This tool (in the SDK bin directory) views the log files from Fusion, the .NET assumbly loader
ieHHTPHeaders Jonas Blunk IE Explorer bar showing the HTTP headers sent and received by IE
LeakDiag Microsoft Leak detection Helps detect memory leaks
MsiInv / MsiZap http://www.huydao.net/ Installation problems List and/or remove MSI-based installations to diagnose/cure setup problems
PortReporter Microsoft Networking/security Logs TCP and UDP port activity
RegClean Microsoft (via PCWorld.com as no longer supported) Registry Helps clean up registry after failed (un)installs
UrlScan Watcher Darrell Norton IIS Watches the UrlScan log files (in %WINDOWS%\system32\inetsrv\urlscan) where the UrlScan component of IISLockdown logs refused requests

If you know of any others, please let me know...