Majic

JIRA is great, but it has a ton of features, and a lot of those features aren't always needed by small development teams.

Majic is a JIRA 5 agile client for Mac OS X. It cuts out all the features except the ones you use most often (and throws in some time-saving tricks as well). Like the JIRA agile web UI, issues are split into columns for "To Do", "In Progress", and "Done".
The program provides full support for creating and editing issues and versions and support for most custom field types as well.