As you probably know there are commercial tools from RedGate, ApexSQL etc. I use RedGate, which is quite good. I am told Apex is very good as well. But, that is not what you asked.
As for free tools, I don't know of any that are as convenient as the commerical tools. You can use something like DiffMerge. It is inconvenient because you have to script the stored procs and save as files and then compare. But once you save the files, DiffMerge is quite good.
I'm not 100% sure about this as I have other editions of Visual Studio insalled on all the machines I have access to, but if you install the SQL Server Data Tools (SSDT) it'll install the Visual Studio Shell. And I'm pretty sure that the VS shell will allow you to do Schema Comparisions.