There are lots of different editors out there and to an extent the choice of software depends greatly on your web development experience. Another factor will be the language you intend to develop in.
Is your current website built entirely in html and if so do you use css either inline or as an external css file.
Do you use a scripting language such as asp, php or perl within your site. This will limit the choices available to you.
What experience level are you at as a developer. The reason I ask this is to work out how much help you will need from the editor. For people with little or no experience it is oftem far easier to use a wysiwyg (what you see is what you get). But this has its drawbacks. You are very much limited by what the software can do, not what you, the designer want it to do.
For experienced users a plain text editor is sometimes a better solution. This allows the designer to have total control over the layout and not be restricted to what his/her editor can do.
There are exceptions to this rule though.. Many modern editors allow you to use the software as a wysiwyg tool or as a code editor. With such software you are able to work on layout in design view, then swap over to code view to make fine adjustments or to correct or optimise the code.
What I suggest you do is download some trial versions such as expressions and adobe and see what works for you. But remember notepad can be just as good. it all comes down to you.
Edited for typos (bad moderator lol)
[edited by: mack at 1:16 am (utc) on Jan. 22, 2009]