I needed to get the site column IDs for some of the built in site columns and while there are several places on the Internet where you can find the technique for pulling them out of the URL when editing a field that was going to be a bit tedious for what I was going to do so I wrote a quick utility that would dump out the site columns to a text file. The resulting XLS file is available here.

I hope that it helps if you're looking for a specific field IDs and you don't want to URL decode them.