I would defenetly get worldedit and worldguard to prevent players from breaking the spawn and other certain areas.
then you need a permissions plugin (if you want to have ranks). I use groupmanager and im very happy with it.
you also want essentials if you dont have it yet. for commands like /sethome /home /spawn /help
towny ofcourse!
maybe you also want chestshops, so people can build their own shops.
thats about it when it comes to starting up your server. if you have a stable playerbase you might want to add plugins like jobs, mcmmo, playerheads.
