Looks like it doesn't support HTTPS, which means many sites can't be filtered by it, and that number will only increase.

I use Proxomitron ( http://en.wikipedia.org/wiki/Proxomitron ), which is quite similar in basic operation but also allows filtering HTTPS using OpenSSL. You do need to create and install your own certificates, which fortunately isn't all that difficult. I suppose you could call it a "benevolent MITM". The author has unfortunately long passed away, and it's not open-source, but there's still a small and active community working on patches to improve its functionality.