Mutualists (symbionts):
fungi that have a mutualistically beneficial relationship with other living organisms.
Mycorrhizae: associations of fungi with plants roots
Lichens: associations of fungi with algae or cyanobacteria
Notes:
Mutualists are very important and very common in nature. More than 90% of plants in nature have a fungus associated with their roots as mycorrhizae. And lichens are everywhere!