Giving something away is harder than you'd think.
Producing it is hard. The material is challenging to master and sometimes even more challenging to present. And the development of the materials is technically hard — Linear Algebra has two hundred thousand lines of intricate LaTeX. It takes time. Years.
It also takes money. That includes paying for this web site, along with expenses such as providing display copies for conferences.
Can you help?
I'm always very grateful for contributions.
If you have a bug report, or if you have something larger, then I'd be interested in having a look. My books have greatly benefited from the contributions that I have gotten over the years. That includes contributions from both instructors and students. Send it to my contact information (I prefer email to git pull requests; the pull request model works wonderfully for software but not as well for books). For instance, I would welcome exams or problem sets, including those for electronic tools such as WeBWorK. Of course, contributions need to be under the same license as the other materials.
Or, you could make a financial contribution. The most common amount is $10 but for whatever you can do, thank you.