FounderDating is like a LinkedIn / social network / discussion forum solely for entrepreneurs and startup founders. The site was made for people to find potential co-founders for their ventures, but also has many other useful features such as their blog and discussion boards.

The Daily Digest email newsletters in particular are excellent - a steady stream of topics from other founders on anything and everything relevant to someone running a startup, with thoughtful replies and debates from other founders. This is by far the aspect I love and use the most.