A quite different approach is to use a different markup for maths, possibly requiring specialised client software. These other notations typically use semantic markup - expressing the structure of the maths. At first sight, this seems preferable to LaTEX's presentational markup, but its weaknesses for authoring are exposed (I feel) when you realise that maths is not as closed and unambiguous a language as computer scientists feel it ought to be. Semantic markup's strength is in interfaces with computer algebra systems, and databases - Abramowitz and Stegun would be ideal in this form! The major dislocation between the two approaches is what makes conversion from presentational to semantic markup so easy. In passing, I'll note that MathML has both a presentational and a semantic variant.
MINSE uses a server to render maths into gifs on the fly. It seems to work rather nicely, but works with its own semantic maths notation.
There is (was?) a project called Euromath, which includes a structured SGML editor. This project included a converter which could transform LaTEX to Euromath SGML.
OpenMath might be a successor to Euromath. It's an EC Esprit project which `proposes to develop standards for the semantically-rich representation of mathematics'.
GELLMU is a LaTEX-like markup language, intended to be easy to convert to SGML. Specifically, it is intended to support maths (and hence conversion to MathML) well.
The following are specifically concerned with maths in SGML, using either MathML or other maths DTD fragments.
WebEQ is a suite of Java programs which implement MathML. It's commercial.
TeXML is a gadget from IBM which converts XML to TEX via a DTD fragment. You transform your XML to an equivalent document marked up in TeXML, which you then separately transform to TEX.
EzMath is a Dave Raggett proposal for producing maths on the web. It uses yet another notation, and converts it to online form using a plugin (no printing, and Windows only, as of April 1999).