LaTeX is a high quality typesetting system, with features that enable it to quickly create beautifully typeset mathematics and other scientific and technical documentation. Indeed, it is the de facto standard for the communication and publication of these types of documents. Best of all, it's completely free!
Below are five fantastic resources for both getting acquainted with, and using LaTeX:
A web-based version of LaTeX -- This is a great place to go first if you are new to LaTeX. Documents can be saved in pdf form, integrating images into your document is a snap, and this particular site allows multiple folks to collaborate on a document in real-time too!
The Not So Short
Introduction to LATEX 2ε (or LATEX 2ε in 157 minutes)
Need to know how to do something in LaTeX? This is the document to check.
A free Windows version of LaTeX to run when you are not connected to the internet.
A free Mac version of LaTeX to run when you are not connected to the internet.
A variant of LaTeX that enables you to add beautiful mathematics to any webpage. (To see MathJax in action, just look at any of the mathematics on this website!)