What does that mean about pay service? Is that now free? Am I missing the distinction between what's on the linked page and the normal webapp monitoring service?
As far as I can tell it is an addition to the normal monitoring? I can download the plugins via their standard dashboard.
Correct, this is a new feature, in addition to all our existing features. This feature is available at all plan levels, but some other New Relic features require a paid plan.
It looks like the main reason to pay is for data retention -- otherwise the free service only gives you 24 hours of data. IIRC that was also the distinction between the previous services New Relic offered, free for 24 hours, pay for more.
Old (?) free accounts did not have full analysis tools - you could not break down slow requests etc. with Java monitoring, only see the average response time over a time range.
Using, authoring, and publishing plugins is free. If you have a paid New Relic account, all this goodness costs $0 extra - same if you have a free New Relic account.