nvALT is one I like. It's fast and geared towards adding new notes quickly. You can type text to a combined search / create bar and either select a previous note or create a new note by pressing enter. All notes are just text files.

That said, I still like to use actual index cards especially because they are slower. Writing text by hands gives time to think what needs to be done and how best to express it. And doodling is fun.