Plugin Suggestions - Wordpress as a Membership Directory
@Nick
You must go with DirectoryPress if cost is not barrier. This is excellent Directory Plugin. Check below link...http://directorypress.net/
If you're looking for Free Plugins then here are few of them...
http://wordpress.org/extend/plugins/business-directory-plugin/
http://wordpress.org/extend/plugins/connections/
http://wordpress.org/extend/plugins/wp-easy-business-directory-1/