There are lots of third-party apps for the Apple Watch but many of them are simply an extension of the iPhone and have not thought out the Apple Watch design and screen.
Of the apps that I've tried so far, the best are the BBC News and Due. Due is a reminders app which is neatly displayed on screen and fills a gap given that the iOS Reminders app is not available on the Watch. It does what it should without giving too much clutter and provides simple notifications when a reminder is due. It is clean and unobtrusive and a useful example of a Watch app. BBC News also provides a few pieces of information with the current headlines and a short summary.
On the other side there are lots of quite useless apps, merely extensions of the iPhone version. Most of the Twitter apps don't make sense on the Watch- they seem to be there as a way to claim they have a Watch app, but with little functionality.