You don't need any tool in the world to do yoga, which is one of the best things about it. You can do it completely naked, in your bathingsuit on a beach, or in a $400 outfit with all sorts of accoutrements. But even though you don't need anything to do yoga (though most people prefer to use a mat), sometimes it's nice to have a little assistance.
Beyond blocks, mats, bolsters and straps, which are de rigeur at most yoga studios, one of the accessories below might well help you take your practice to the next level.
A round block can be used just like a square one (for balancing postures), as well as a stretching tool (rolling it gently away from you in seated forward bend, for example), or even as a back or leg massager post-practice. The rDot
from Yogitoes is made from rice husk and EVA and dyed with metal-free dyes.
Grippy Toe Sox
are easier to carry and clean than your own yoga mat, and mean that you'll be ready to jump into class using whatever mat is available, or even without any mat at all (perfect for hotel gyms on the road).