Most of my experience as a spinner has been with protein fibers. They lend themselves well to the finished items that I like to create, but the spinning world is oh so much more than just protein fibers. Plant derived fibers offer a wide variety of choices for spinners, and a whole new set of preparations.
Cotton is somewhat readily available through online fiber shops and comes in a variety of different forms. It can be spun from punis, which arrive wrapped in bundles (usually Indian newsprint). Cotton punis are ideally spun on a charka, which allows for a great deal of twist to be imparted to the fiber. They can be spun on a wheel, just be sure to rig the wheel to its highest ratio possible to save yourself some frustration. Cotton can also be prepared in a sliver or roving. This is usually a good indication that is has a longer staple length. While it is still best to use a tug test to see exactly what the length is, it does give you a starting point to search within. A tug test is simply tugging a few fibers from the end of the roving to see how long they are. Also examine to see if they are all the same length, or if there are some short pieces included in the roving. These will create neps just like second cuts do in wool.
Cotton also comes in a variety of natural colors. Naturally, white is the most frequently found. But I was surprised to find there are natural greens, rusty oranges, camel browns and even chestnut. The colors give you the chance to spin the cotton without committing to a whole new set of dyes.
We are also hearing about renewable and eco-friendly fibers like tencel which is derived from wood pulp, seacell which is seaweed bonded to a cellulose base, and of course bamboo. As you consider these fibers, look in the descriptions for how they are processed. Sometimes the processing creates more pollutants than other more traditional fibers. They will spin lovely yarns that will have a sheen to them unlike any other fiber. The bamboo is a personal favorite of mine for blending with other fibers.
Flax comes in numerous preparations. It can be purchased in a strick, roving, line, or even combed top. Flax is a spinning experience unto itself, as the fiber is so much longer than any other. Most flax has at least a 12 inch staple length. In roving form, pre-drafting is essential, otherwise you end up with an anchor rope. The finished products from flax are lovely, lasting and certainly worth the wrestling match at the wheel.