BBEdit is a popular, powerful HTML and text editor with a high-performance array of tools for editing, sorting, searching, transforming, and otherwise manipulating text. Anyone with modest text-editing needs should check out BBEdit's free sister app, TextWrangler, but BBEdit is still an excellent choice for software developers (especially Web developers), with potent hand-coding features across more than 20 programming languages.
On top of basics like code folding and syntax coloring, BBEdit has flexible, multilanguage code completion, nonmodal search and replace across multiple files (so you can keep multiple document and search windows open), a smart Scratchpad that automaintains its state and contents, multiplatform Web browser preview, and--as they say, but in this case it's particularly apt--much, much more. The most recent update adds a Sleep command, support for Growl and LassoScript, and improvements to expert preferences, MobileMe syncing, Terminal integration, FTP implementation, and more.
BBEdit has certainly attracted more competition over its many years--and you can find pieces of its functionality in a variety of cheaper apps--but this app remains popular for a reason, with its rich, robust feature set and tons of options for customization.