The Yoga mat is perhaps the most fundamental of yoga props. It is essential for providing traction under the feet when doing standing poses and general cushioning under the body for floor work.
Mats come in all sorts of colors and textures. Some provide more traction than others. If your mat is slippery when you first get it, run it through the washing machine a couple of times to break the surface down a little. If you tend to sweat a lot, then many yoga prop suppliers sell cotton dhurries, a type of Indian rug that provides excellent grip when wet.
If your mat starts to lose its traction and begins to smell from long use, you can clean it by running it through the washing machine. Use a gentle cycle with cold water and take the mat out before the spin cycle if possible. You probably will not need to use detergent, but if the mat is heavily soiled, then use a very small amount. Either drip dry or run the mat through the dryer at VERY low heat.
The standard mat for many years was produced by HuggerMugger. Their "Tapas" line of mat is sturdy and pliant and comes in many different colors and retails at around $20.00. One criticism I often hear of this mat is that it's surface texture can be slightly "oily" and some people have trouble with traction. It comes in different thicknesses, 1/8" (standard) and 1/4" (thick).
There are also several versions of a thicker, much heavier mat that provide excellent traction and cushioning, without being too "bouncy". The original, and the rolls-royce of these, is the Manduka Mat from Germany. It is very heavy, about seven lbs., but worth the extra effort of carting it around if you are serious about your practice.
HuggerMugger also do their own version, the Mandara mat, which is a pale comparison to the Manduka, but still quite durable. It comes in the regular, 8lb. size:
...And the "Mandara Max" 10 lb. version:
My personal favorite is a much thinner (2mm) mat available from Tools For Yoga. It offers less cushioning, which can actually be a bonus for those with wrist issues, and is much less cumbersome.
(click here or on the image to go to the product page.)
Also very popular amongst some of my friends is the "Harmony" yoga mat by Jade. It is made of latex rubber and is biodegradable. It is a thick, heavy mat with excellent traction. The only criticism I have of the mat is that it is too "bouncy" for some poses. Doing headstand on it is like doing it on a boat in a strong head wind. A small criticism for what is an otherwise excellent product.