Keeping dependency versions up to date in Leiningen projects

The canonical way of doing this, at least for dependencies kept in clojars, is the lein-search plugin.

Update: See the highest-rated answer below for a more up-to-date response.

The Clojure ecosystem has evolved since the original answer was offered. At the present time, I would recommend using lein-ancient:

A Leiningen plugin to check your project for outdated dependencies and plugins. This plugin supersedes lein-outdated and uses metadata XML files in the different Maven repositories instead of a Lucene-based search index. Version comparison is done using version-clj.

Its precursor, lein-outdated, has this helpful message in its README: "lein-outdated is outdated". :)

You should have a look at the answer to this question. Leiningen uses the same versioning mechanism as maven so, for example, if you want to use the latest version of a given library, you can substitute the word "LATEST" for the version number. You can also specify a release version or a version range. Again, look at the answer at that link.