MathML, VoiceXML, etc.

MathML MathML provides a set of tools for including mathematical information (equations, matrices, etc.) in Web documents.
Platform for Privacy Preferences (P3P) More a supplement than a presentation format, P3P uses XML to communicate privacy preferences between browsers and servers, negotiating acceptable use of information. P3P has been a long time coming, and there isn't much sign of popularity.
XForms XForms supplements XHTML and other markup vocabularies with a much more powerful set of interactive forms than HTML provides. It's still on the horizon.
Small devices WAP's Wireless Markup Language has been one option, but a subset of XHTML, called XHTML Basic, seems to be growing more popular.
VoiceXML VoiceXML provides a very different option for presenting information interactively through sound.