I've been using nvAlt for a while. Very fast search and ease of use. Not sure how well it scales in the long run but thus far I've had no problems.

Edit to add: Some other interesting features. You can sync to Simplenote automatically, and notes can be saved as text files for easy import/export. There is some markdown support (though I see a number of bugs in the preview). The main downsides appear to be: limited color schemes, no syntax highlighting for markdown, development appears to have stalled.