Plugins are a very useful technique and can be used to lots of different things. But sometimes things don't work like you expect them to...
Here at Logica Sweden we have developed a CRM Framework with lots of nice tools to aid your everyday CRM development. The architect behind this hasn't been me but my fellow CRM architect Daniel Halan. This framework is contained in a dll and it is used in plugins.
The problem I was facing was that when I updated the framwork and rebuilt my plugins, to the assembly directory, it also put the framework dll in this directory. But when I tried to run it I got the most weird errors saying that it could not find methods, constructors and similar. If I installed the framework dll in the GAC then it would all work fine. This is something that I would like to avoid, however, since it is a bit cumbersome when developing.
After some collective debugging we found the error, plugins don't use the dll:s in the assembly directory, but the dll:s in C:\Program Files\Microsoft CRM\CRMWeb\bin (or similar depending on where you have installed CRM).
So, make sure you put your referenced dll:s in the correct directory and happy coding!
Microsoft Dynamics CRM Architect