I recently needed to add a reference to a web part where the dll had not been strongly named. The result of doing this was a build error complaining about the unsigned dll. I did not have access to the source code so couldn't resolve the issue by rebuilding the dll with a strong name. Instead I used the ILMerge tool which can be found here.

Rather than using the tool for it's intended use of to combining a number of dlls into a single dll I used it to sign my single dll using the following command line syntax:

ILMerge.exe /out:c:\signed.dll pathToFile\unsigned.dll /keyfile:pathToFile\aidangarnish.snk