I use Jeffrey Friedl's “jf Zenfolio” plug-in and highly recommend it. Unlike most plug-ins of this sort, it get syncing right. Most will allow you to upload once, and then if you change things in Lr, the syncing goes to heck. Not JfZenfolio. It's a pleasure to use.
I second that recommendation. The plugin author has an excellent reputation, and I'm very satisfied with his Flickr and Zenfolio plugins.