For coding html, css, php, js etc coda is one editor that I hear a lot of people using on mac, but it's not free. A free alternative would be aptana studio, if your familar with eclipse you will like this.
For design I assume you have Photoshop?
Design: Photoshop, xScope, Layers, Balsamiq Mockups
Development: Coda, Changes, GitX, SQLEditor
I used to use skEdit but seeing as there's been no release in quite a while I've moved over to Coda which seems to be improving a bit faster.
What are the benefits of Coda over skEdit or is it just personal preference? I found skEdit much easier to use than Coda and editing remote files/syncing local/remote to be a big benefit.