Lifehacker has all the resources you'll need for using Markdown.
There's hundreds of apps out there that are prepared to lock in your data. But why sacrifice access to your data? Look at moving away from these apps that lock in your data and start using plain old text files for your to do lists, logging, writing and blogging.
Plain text and Markdown is much more flexible and you can use a wider range of tools to manage all your Markdown documents. Add a cloud storage service like Dropbox and you can have your files with you everywhere you go!