I am looking into systems which allow people to get all the benefits
of the Internet, yet block their children (if they wish) from getting
to inappropriate sites.

What are the top 10-15 Internet Content filters? (i.e. Net Nanny etc)
How many (approx) users use these systems?  What are the main
differences between these systems?

The main players are ContentWatch, CyberSitter, Cyber Patrol, Net
Nanny.  The thing you should look for is the ability to overide any
blocked site if you are the administrator.  No filter is perfect, so
it is important to have this functionality.

I've tried most of the products and I like ContentWatch the best. 
Their filtering is really good and they have reporting tools so you
can look at your kids Internet activity.