A tester is often only as good as the tools s/he knows how to use. Here are some of my favorites:
- Camtasia Studio - Video recording for your testing sessions. Free trial version. Around $300 for a single license. Totally worth the money.
- Fiddler - Tool for monitoring the details of HTTP traffic.
- Hyper-V - If you're not using some sort of "virtualization technology", you're severely handicapping yourself. I use virtual machines (VM's) like crazy to quickly test against different configurations of OS's and software. Just make sure you have a host machine with tons of RAM.
- OneNote - Microsoft's OneNote is a great tool for taking testing notes. Nice collaboration features so multiple folks can work on the same page and a handy built-in screenshot utility for injecting those into your notes as well.
- Process Explorer - Great utility for seeing all of the processes running on the system, handles opened and other general process monitoring goodness.
- Reflector - An invaluable tool for disassembling and analyzing .NET Framework binaries. I'm particularly fond of the "Used By" feature which helps me quickly navigate dependency chains.