I use .md for my Markdown extensions. I know Gruber prefers .txt, but I like having syntax highlighting and the ability to process batches of them in a folder mixed with plain text files.
Today I learned Vim (at least 7.4.769) has syntax highlighting for Markdown, and is activated with the .markdown extension. I like to keep Vim fairly generic, so that’s enough for me to start using it for everything.
Now I just need it for the IBM E Editor and I’d be set.