This userscript allows non-Premium Members to log Premium Member Only (PMO) geocaches on Geocaching.com
You might wish to do this for one of the following reasons:
Once the script is installed, on visiting the cache listing for a PMO cache, you should see a new "Log your visit" button on the right of the page, above the advert. The script does not reveal the coordinates, description, clue or anything else that is normally hidden from ordinary members.
Please note that some cache owners may not appreciate logs from non-Premium Members, although it rather depends on the owner's motivation for making the cache PMO in the first place. If in doubt, ask first or explain your situation in your log post. The technique used by the script has been kicking around the Geocaching Forums since at least 2007 and seems to be tolerated - if not exactly promoted - by Groundspeak.
If you are new to userscripts, there is a HOWTO with instructions on how to get started.