Robert_Charlton - 8:26 pm on Oct 2, 2013 (gmt 0)
Pjman... strictly speaking, the Google Search Forum Charter [webmasterworld.com] rules out posting specific search terms, but I'll allow your suggestion in this case. ;)
My guess is that a home rolled solution might be better if you're proficient enough to build one... but that some of the packages might save a lot of time. Also, it appears, knowledge of php would be helpful.
I searched within site:webmasterworld.com using a variant of the title to the current thread as search terms, and I recommend the search as a start to reading on the topic.
Here's a thread that leads to some suggestions Google made when the feature was launched....
Google Clarifies "First Click Free"
They also have some implementation suggestions (i.e cloak for googlebot, show protected content based on the referrer).
Note that cloaking for googlebot isn't something you'd normally expect Google to recommend. See the above thread for more discussion on this.
If anyone does have more detail thoughts on implementation, they would be helpful, as we don't have much detail on this aspect of the topic.