I do a lot of work which needs bits and pieces of HTML. Generally I paste these into Ranchero’s MarsEdit for a quick preview.
Andrew’s Widgets SnippetComposer widget offers another option.
SnippetComposer is for folks who know some HTML and want a way to quickly compose HTML snippets, for example, for posting to online forums.
Type or paste HTML in the top input box and SnippetComposer will show the rendered HTML in the bottom box. When you are ready, click the C button to copy your HTML to the pasteboard. Click X to clear the input fields.
It’s not really big enough for my needs, but maybe it’ll be useful for you. By the way: pasting in the text isn’t enough — you need to press Return too.