The new carnival is up at mathfuture. One of the most interesting things about it is the host site, which looks at Web 2.0 math applications. For example, their GameGroup, which I may be seeking to join.
Other than that the 2 posts that were most interesting to me are a description of a role-playing site for math-centric careers and 25 uses of Wolfram Alpha (which I love).
Denise has a good starter on Mental Math, but I don't see these as techniques you teach as much as ways of thinking that you demonstrate and can grow out of kids' number sense. I was saying to Xavier (my 4th grader) last night, knowing 6x9 is not as important as being able to find it efficiently, and knowing things like it's connected to 6x10 - 6 or 6x5+6x4 is much more important.