For the Macintosh
BBEdit
Not free. In fact slightly pricey, but definitely worth every cent. Great for all script wrinting.. php, asp, html, xml, unix, etc. If it's too costly for you check out
TextWrangler, made by the same company.
SEEdit
Shareware and fairly inexpensive. Nice light code editor.
CSSEdit
Shareware. Nice visual CSS editor. Lets you see your styles as you edit them and contains many shortcuts for developing style sheets pages.
XScope
Shareware. Not really so much for web development as it is for visual checking. it allows you to use frames on your desktop to see the screen real estate every browser will use at certain resolutions. This allows you to check pages without having to change your actual monitor resolution.
Transmit
Shareware. Probably the best FTP client for the Mac.