Lurch
Your word processor has spelling and grammar checkers. Your math word processor should check your math. (Not just arithmetic, but algebra, calculus, and proofs as well.) That is what Lurch does; it is a simple math word processor with a general-purpose math checker built in.
The project began in June 2008 and version 0.8 of our desktop application is currently released. I am coauthor of Lurch along with Nathan Carter of Bentley University. We were supported by a grant from the National Science Foundation from Summer 2008-Summer 2011 (grant #0736644), and continue to work on the project. We use Lurch in some of our own mathematics courses at the University of Scranton and Bentley University.
You can read all about Lurch at lurchmath.org. You can download it free for Windows, Mac, or Linux.
We are currently in the process of redesigning Lurch completely so that it can be used in any web browser. This would eliminate the need to install the software as a desktop application. You can read about the status of our web browser based version at lurchmath.github.io.
If you have any questions about getting started with Lurch, please contact us.