Every developer has run into this issue at least once. There are a few different tools you can use to find out what process is locking a file. In the past I've used a few different tools.
My favorite tool today is Lock Hunter which is powerful and easy to use. It's packed with a bunch of useful features, integrates nicely into explorer and installs without any "bonus" software. I can't recommend it enough.
Before Lock Hunter I used
Unlocker by Empty Loop as my go to solution but today it's bundled with a toolbar and isn't early as pleasant as Lock Hunter.
If you're already using Process Explorer it can be a great solution. You can use it's
Find Handle or DLL to locate the culprit. On the downside (or maybe the upside depending on your point of view) there are is no explorer integration.
Handle is another tool by Systernals but aimed solely at the command like. This tool focus on the basics and leaves the action up to the user.
Missed a tool you love to use? Leave a comment!