Mendeley has received a range of incremental upgrades and tweaks over the last six months. These include a redesigned interface, better PDF handling and searching, an improved plugin for MS Word on the Mac, and refreshed citation styles.
The most interesting advances, however, deal with finding a easy way to edit or create custom citation styles. Mendeley partnered with Columbia University Libraries to create an accessible editor and you can start using the beta at http://editor.citationstyles.org/.
The editor allows you to create your own style, tweak existing ones, and even enter what your style looks like and find matches from the database. While the system is coming along, it is still not the most intuitive, and I hope that they continue to refine. That said, it is a viable way to edit and create new citation styles for Mendeley or Zotero, something users have been asking for.