I'm from an AS/400 (iSeries) background and have no idea about how a team of programmers work within the .Net environment.
Lets say a company purchases a new button control. Now, in that company they'll have 5 or 10 developers working with that new control. Its my understanding that even if the control is in the GAC, it must still be copied locally to work on at design-time.
My question is, what is locally? Does each developer have a copy on their own PC or is it held centrally on a server?
local on every machine. Which means each pc has the control installed on it. When you are ready to deploy you can use a server for your end users through click once deployment.
In my old company we usually used DevExpress control libraries, and the whole thing was pretty straightforward. We would just run DevExpress installation package on each computer and the controls would be automatically added on Visual Studio toolbox. Then, you just needed to drag and drop them on the designer and start coding :).
Third-party libraries are local on every machine. We use a SubVersion source control repository that includes a /lib subdirectory containing all third-party assemblies, referenced as necessary in local projects. If a library changes or is added, that means one developer will have already set it up and gotten it working, and all other developers will get it on their next SVN update. As our project is ASP and we don't think much of the VS web designer, there hasn't been much need to customize the Toolbox, but in the case of a WinForms app, you can add them manually by right-clicking an empty part of the Toolbox and selecting "Choose Items...". The solution is set up to properly deploy everything to a dev, build, test or UAT box on a build of the solution (we just substitute some config files depending on the exact environment).