In every day Sharepoint development we often need to update assemblies in GAC via WSPBuilder. Sometimes (especially if you use ReSharper) you will see access denied exception which is caused by the fact that assembly being updated is locked by another process:
Sometimes adding assembly 2nd time solves the problem, sometimes not. In the last case you need to determine what process blocked your assembly. In order to do it you can use GUI procexp utility (Ctrl+F to find dlls attached to process) from Sysinternals. There is also built-in Windows utility tasklist which allows you to find what processes use specified dll assembly:
This little tip may help you not only in Sharepoint development as well.