No matter how casually you ride, every MTBer should invest in a helmet. Mountain biking requires a higher level of protection than most other cycling disciplines, and a specific mountain bike helmet is best. More serious riders should consider a full face MTB helmet, which offers protection for the jaw and nose, and body armour.
This is helpful, given the likelihood of taking a hit to the torso and legs is high in this aggressive (but oh-so-fun) sport. Downhill helmets are a step up again, differing only slightly from motorbike helmets in their construction.
Many of the helmets we stock accommodate MIPS technology, which allows the head to move slightly on impact, reducing rotational forces. Fox, Bell, and Giro are some of the most respected names in helmet technology. Check out their MTB range.