I am including a list of some of the sites that I have found particularly impressive. I'm always keen to learn about new sites and if you want to let me know about high quality treasure hunt sites, you are welcome to email me using my contacts page.
In no particular order:
Quest4Treasure is created for treasure hunters and puzzlers with three main areas of interest: armchair treasure hunting, real treasure hunting and puzzles. This is an enthusiastic site with a well used and comprehensive bulletin board.
The site defines CryptoVoyaging as "The search for deliberately hidden secrets that takes the seeker on a physical or virtual voyage." The site is designed "as a launchpad for 'Cryptonauts' everywhere..."
Tweleve is a very large, friendly community. Lots of links to great armchair hunts and spade hunts.
The Armchair Treasure Hunt Club was founded by author Dan James, famous for his work in the art of illustrating and producing treasure hunt books. For a small fee, members receive club treasure hunts and puzzles to solve, plus news from the treasure hunting world.
HadToPlayOn "are a very friendly and growing community who have one thing in common ... our love of puzzles. We hold regular free contests based on a wide variety of topics such as movies, music, traditional riddles, wordplay and much, much more."
Since setting up The Tenth Leaf I have discovered a number of vibrant, active sites that will be relevant to you if you have an interest in keeping your brain active, having fun, and enjoying the company of other like-minded people.
Treasure hunting is a popular hobby with a sizeable international following. Treasure hunts come in a variety of flavours including geocaching, scavenger hunts, spade hunts and armchair treasure hunts, to name but a few.