Zach Schneirov, who maintains the wonderfully minimal Notational Velocity notes app for Mac OS X, has added full support for Simplenote syncing. You can get Notational Velocity for free or read about how his implementation handles various syncing scenarios to provide a very robust experience.
A lot of people have been anticipating this since we announced it a couple months ago, but we think this layer of extra polish has been worth the wait!
Since Notational Velocity also supports seamlessly storing notes as text files in the directory of your choice, you can also use it with Dropbox to synchronize your notes across a variety of systems. Just point Notational Velocity towards a Dropbox directory and you’re all set.