TiddlyWiki lets you write ordinary HTML by enclosing it in {{{}}} and {{{}}}:
Link to wikiwords from HTML
The source for the above is:
{{{
}}}
HTML can enable some exotic new features (like [[embedding GMail and Outlook|http://groups.google.com/group/TiddlyWiki/browse_thread/thread/d363303aff5868d0/056269d8409d121f?lnk=st&q=embedding+gmail&rnum=1#056269d8409d121f]] in a TiddlyWiki). However it doesn't work for some JavaScript code libraries; see CustomMarkup for another way to include custom HTML in your TiddlyWiki.