The desirable features for an html editor are syntax coloring, code completion and hints during the editing.
In the past i've used TextWrangler (Mac) for testing code written in some other language (C, Fortran, php): it has syntax coloring for a lot of different languages, it is fast and powerful, and it's free.
Now, i've just tried Mozilla Thimble and i think it's cool because i can immediately see the results of my code and it has all the other desirable features i've listed.