At my current project for Avanade I've been working with Hisham (who is also my Career Manager and whose blog I read before I even knew about Avanade) and we've been doing a lot of BizTalk stuff. As he stated here, BizTalk development, without the help of some external tools, can be very time consuming just in the build->deploy->bind->enlist->start process of running BizTalk. One of the pieces that I've been working on is a custom Flat File disassembler. I found that testing my component within BizTalk was very time consuming. I also knew that there was a utility in the SDK for testing flat file disassembly (FFDasm.exe) which uses the standard flat file disassembler (FFDasmComp). So, with a little help from Reflector, on my flight to Seattle I hacked together a new version of the utility (FFDasmEx.exe) which accepts an additional parameter for the name of the custom pipeline component that you would like it to use instead of the standard disassembler.
This makes testing a breeze as you can simply compile your pipeline component assembly, copy it to the Pipeline Components folder, and then run your test. You can easily debug your component using this same method but running the FFDasmEx project in debug mode.
If anyone is interested let me know and I'll zip up the project and post it somewhere. However, if you have a copy of Reflector you can probably put it together yourself in about ten minutes.