Nut shell or hull (hard bean shell) mulch is becoming hugely popular with those who are environmentally minded. Not only is it relatively inexpensive, it aids in plant growth by being high in nitrogen. This mulch will eventually break down (releasing the nitrogen into the soil) and will have to be replenished, but this is normal for any natural mulch. Nut shell or hull mulch also provides a beautiful aesthetic to any garden or walkway, especially when the shells are left intact instead of shredded.
The misunderstanding about tree bark mulch is that it causes more trees to be cut down. This is not so. Tree bark mulch comes from responsible logging companies who understand the need to use all parts of the trees they cut. The bark is stripped from the logs and turned into mulch. It is a forward-thinking use of resources.
Possibly the most controversial alternative to gravel is rubber mulch. Rubber mulch is made from shredded tires after the steel content has been extracted. Although not biodegradable, rubber mulch is an excellent way to reuse what could otherwise become a bio-hazard or landfill problem. Rubber mulch is best used for walkways and playgrounds. Chemicals remaining in the rubber could potentially damage or even kill certain plants.
Invasive tree species, like Australian Melaleuca, are damaging other native tree species like the cypress. Mulch companies have begun working with national and state environmental agencies to eradicate these invasive species and turn the trees they cut into usable mulch. Eventually this resource will no longer be available once the removal is complete. However, by the time this happens, the hope is that native species will have been revitalized to the point of being responsibly harvested again.