When I came to Microsoft I saw a really cool tool being used internally and immediately wanted to work with the author, Rocky Downs (who is blogless, but exceeding talented), to get it released. Fast forward to now, plus an installer, and here it is.
The basic (as in "only") idea is that RockScroll extends the scrollbar in Visual Studio to show a syntax highlighted thumbnail view of your source. This is really useful for those excessively long source code files you know you have. It's just one DLL and you can turn it off from Tools|AddIns just by un-checking the checkbox.
Works On My Machine Disclaimer: This is released with exactly zero warranty or support. If it deletes files or kills your family pet, you have been warned. It might work great, and it might not. It hasn't been tested against the myriad of other VS Add-Ins, but it works on my machine in both VS2005 and VS2008. It does look a little odd next to Resharper, a tool that also adds a scrollbar. Good luck.
A reader, Hans, says: "I have found that (not suprisingly) RockScroll works very well with split windows, if you bind a shortcut key to the "Window.Split" command." for those of you having trouble with split windows.