An interactive problem website for Clojure beginners (here, via @hnfirehose) -- Written in Clojure and MongoDB, 4clojure is an application that presents the user with problems to be solved with some clojure code. This is how you start testing your clojure knowledge.
Clojure Atlas (Preview): An experiment in visualizing a programming language & standard library (here, via @ClojureAtlas) -- Chas Emerick created a service that provides a way to visualize your clojure the language and its standard libraries. Check it out, it is very creative. The project is in early stages, and if you want to try it, enter your email here to get a notice when it is up for user testing.
Chasing Erlang: profiling a Clojure library (here, via @planetclojure) -- Antonio Garrote, the author of Jobim, an Actors library for Clojure which is modeled after Erlang, wants to know how programming Clojure with Jobim compares, performance-wise, with Erlang itself. For this test, he had to explore how to profile clojure and find hot spots. This article covers both the profiling part but also the full benchmark.