See also Lisp_editing for other lisp plugins, like the excellent Parinfer.
CIDER is now the de-facto Clojure IDE on Emacs.
You can see videos, like this one: getting started with Cider.
It is in Melpa.
Helm-clojure is an earlier project with similar ideas (see the gif inside).
Most Starter Kits come bundled with Clojure support.
Use cider-spy to send code snippets and text exchanges to each other etc, as well as EMACS bookmarks.
yasnippet snippets for clojure (in MELPA).
ac-cider is a completion source for Emacs auto-complete package that uses CIDER (and Compliment) as candidates provider.
clj-refactor is a collection of simple clojure refactoring functions. Available in Marmalade and MELPA.
The clojure cheat sheet for emacs packaged into an easy, fast, searchable, offline form.
Typed Clojure mode
The official Typed Clojure Emacs minor mode.