One criticism repeated often in the Baltimore sessions was the paucity of exercises in open-source texts. Authors uniformly indicated that providing a carefully edited and complete set of exercises (including solutions) for their texts was the most demanding part of creating the text. For some texts at the pre-college and early college levels (algebra, pre-calculus, and calculus) the MAA's free and open-source WeBWorK on-line homework system can help to fill this gap.
A couple of years ago, I decided to give WeBWorK a try in my calculus II class. Since it's free (I'm a cheapskate at heart) and I have a few computer servers at my disposal, I downloaded the instructions for installing WeBWorK from their Get WeBWorK page. In particular, I had an Ubuntu 12.04 linux machine, so I downloaded step-by-step instructions for an Ubuntu 12.04 system. I will admit to having a fair amount of unix system administration experience, but if you're passingly familiar with unix, you can probably get through this in an hour or two. If you're not a unix person (and not a cheapskate) you can actually buy access to a WeBWorK server at a relatively low-cost.
Long story short, I set up my own server and proceeded to use WeBWorK to provide students with additional exercises. I taught out of Guichard's Calculus text (free, open-source). Guichard's text has a decent number of exercises, but WeBWorK had far more to choose from. In addition, WeBWorK provides instant feedback which students indicated they liked. Finally, with WeBWorK the instructor can closely monitor whether students are actually doing homework and whether they're doing it on time. (Can you tell I'm a fan?)
Back to the point of this post, open-source authors faced with the daunting task of creating exercises and solutions for their texts should consider pre-existing open-source repositories of exercises, like WeBWorK, to fill this gap. IMathAS is another open-source homework system for mathematics (but I haven't used it). If any of you know of other open-source collections of exercises for mathematics, please post references in the comments.
If any of you are teaching calculus I, II, or III out of Guichard's book, I have since created WeBWorK problem sets that correspond to nearly every section of the text. You can download the .def files for calculus I, calculus II, and calculus III. They may not perfectly fit your needs, but they certainly provide a good starting point for your problem sets.