Most mathematical writing these days is done with LaTeX, a professional-quality typesetting program that is available at no cost for all common computer platforms. There are lifetimes' worth of material on the web; the links below constitute an aggressively culled introductory list.
"Why should I care about software freedom (whatever that is)?" To answer this question, you only need ask yourself how software is used in modern society: To conduct financial transactions at all levels; to create, transmit, and archive government documents and other matters of public record; for personal communications (telephone and email) and entertainment (music, movies); to store and access personal information (government, medical, and academic records; credit histories; your web browsing habits). Information is constantly collected about you, whether you are browsing the web or making a purchase with a credit or ATM card. This data is stored and transmitted over computer networks, through channels that are vulnerable to surveillance (clandestine copying) or hijacking (insertion of fabricated data). Electronic data that concerns you directly (news releases, texts of laws, consumer profiles) is often stored in proprietary formats, making this information inaccessible to you unless you have appropriate software.
Software advocacy generally revolves around issues of freedom and access to information: Public records should be available in non-proprietary formats. Information about private individuals (medical and credit records, telephone conversations, email, purchasing habits) should be safeguarded against theft (eavesdropping or copying) and tampering. Laws governing electronic data (such as UCITA or the DMCA) should not be passed (or enforced) if they substantially undermine Constitutional freedoms. Software should not have the legal status of a commodity, but rather be regarded as a form of speech. The inner workings of software should be open and documented, just as food must be accurately labeled.
Obviously, different parties have divergent views on software freedom. Large software vendors often view software freedom as antithetical to their own existence. Governments are reluctant to allow citizens full access to strong encryption, which allows message contents to be hidden even if data is captured. Organizations such as the College use non-free software and data formats because they are nearly the de facto standard. Hackers, people who are curious about the inner workings of technology, often view software freedom as one of the basic rights of an open, democratic society. Most people have little or no idea that these issues even exist.
The links below are in no way comprehensive, but provide a sampling of views and resources. As citizens of an open, democratic society, we all have the right--and the responsibility--to become informed about software freedom.