I've had a little poke around on the 'net. MathCast looks pretty neat - although its final result is an image, but it's smooth using! The other one I saw but didn't check was Mathyma with a range of webpage apps from basic stats, graphs and equation editing (although the site itself suggests the equations are somewhat quick and dirty)
DragMath (http://www.dragmath.bham.ac.uk/) is an OpenSource Java applet we have tried integrating with Moodle. In our circumstance, the OpenSource licence allows all students and teachers to insert equations in place via the Moodle editor (without excess licence fees).
For teacher preparation of online material - assignments, quizzes, etc - we use the commercial product MathType. We have integrated it with MS Word and use it instead of the less sophisticated MS Equation editor. We can copy the LaTex coding from MathType equations into Moodle and have spent some time simplifying / automating this process.
I was actually just going to mention LaTeX. I have not used it to create equations but I know it can. My suggestion would be OpenOffice Math but I haven't really used that either. I just think it could be a bit of an easier learning curve over LaTeX but then again LaTeX might be a good thing to learn.
It kind of depends what you want -- if you want WYSIWYG, previous suggestions for the equation editor in OpenMath are probably right. OpenOffice is generally good for document editing, essentially an open source clone of MS Office. If you are looking for hard-copy output, this should be fine.
If, on the other hand, you are looking for output you can paste into web pages, there are better ways to go. Equations in OpenOffice math editor can be exported, but output is either an image or some really kludgy HTML.
Thinking Web 3.0, though, you are eventually going to want to look into MathML for web pages (not appropriate for hard copy). The problem with MathML is that it is **VERY** verbose -- almost to the point of non-human-readability. I have written a little Java applet for translating a LaTeX-like equation markup into Content MathML, which makes the initial markup much more readable. Then just copy and paste into an XHTML file, and there's your equation. Your students would need a MathML-capable browser, though -- either Firefox (MathML is native) or Explorer (with the MathPlayer plug-in from Design Science).
I kept using MS Equation Editor. I found Open Office too similar and I would need install it on the several computers I use. The other programs (I remember trying one and being scared off) had too high of a learning curve. I'm a science teacher by trade, not math, and simplicity is key. Perhaps in the future something will come up.
I just read in eschool news about some free new Microsoft tools for students. A Microsoft Math add-in that enhances Microsoft Word 2007 with computational and graphing capabilities. With the add-in, users can plot a function, equation, or inequality; solve an equation or inequality; calculate a numerical result; and simplify an algebraic expression. Users also can employ a linear format for entering equations into Microsoft Word 2007 and Microsoft Math.
I have no idea if this would help but here it is! N
I have not found anything that is easier/better than the Microsoft equation editor. It gets even worse if you want to use equations on the web. I have a class wiki for math concepts and adding equations is a major hassle. See this page if you dare... http://klhtfaig.pbwiki.com/Creating+Equations
Equation editor is good for me. it helps if you use short cuts.... i'm collecting the shortcuts i found out by trial and error. i'm using a macbook in school. so my shorcuts are for mac version. any other mac users here? we can share our shortcuts to make typing equations faster.