It's no mystery that mobile users need note taking apps. And that advanced mobile users want to keep in synch their notes between different mobiles, laptops, iPads... Yet, I have always found it difficult to find a good application offering these two features.
Evernote is the 800 pound gorilla in this space. Started with some gorgeous desktop applications, it has rapidly expanded into the mobile market with clients for iPhone/iPad, Android, Blackberry... and adding a ton of new features. Too many, in my opinion (add to that the a sort of "Evernote App Store" called The Trunk). And honestly, after a whopping 20M$ Series C funding, can you still ask it to be a lean and simple note taking app?
That's why I've now turned to Simplenote. Recently launched by a YCombinator 2010 startup, it takes a refreshing approach to note taking (simplify!). Very limited feature set and an API to let developers develop their own clients (note that you need to signup to have access to the APIs).
Besides the iPhone/iPad clients, provided by the same company, many other desktop and mobile clients are available.
Simplenote is a freemium, with a premium version offered at $12 per year. The premium version feature set is frankly of very little interest right now, but it's not difficult to imagine it will be enhanced in the future.
In summary, even if it's a very young tool (coming from a brand new startup), the outlook is positive. Being part of the YCombinator community is giving them popularity among the hackers, so that it's easy to anticipate the creation of many 3rd party tools leveraging the API. On the other side, their focus on simplicity will certainly convince many users to switch from more complicate tools.