There are a few sites that I pay for. It will depend on various things including how much I value the service they offer, how much they charge, whether the same service is available anywhere else, how often I will use the service, and how else they are making money (ads, etc.).
http://www.geocaching.com
Why? This is the most popular site for geocachers. They are the de facto official geocaching site, and have caches that players do not list on other sites. They offer a free membership, but paid members can take advantage of some extra services that I think are worth $30 a year. I visit the site several times a week
http://safari.informit.com
Why? They offer online electronic versions of technical books. You can also print chapters for offline reading. The basic membership is $15 a month. Since most tech books are in the $30-$70 range at a bookstore, I find this to be a reasonable alternative. I visit the site several times a month.
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