As motorsport marshals we volunteer for a variety of events. This is a fantastic phenomena as it never gets boring working different races. Over time people tend to develop preferences and various levels of enthusiasm towards specific venues and luckily there is something for everybody when you get involved in marshalling.
Some marshals are lucky strictly based on their location because they get a great deal of different motorsport series racing nearby, others like myself have to travel to get to the good stuff. But as long there is a will there will be a way.
For fans of open wheel racing there is of course Formula 1. F1 circus travels around the world over a twenty race season. Chances are they will make an appearance somewhere near to your home track. Volunteering for F1 typically requires some experience and a following of a very strict schedule. In North America, Brazil and Australia there is another popular open wheel series called IndyCar, in many ways it resembles the rules of F1.
One of my favorite forms of racing is sports car/road racing. The Le Mans prototypes mixed in with GT fields, sometimes comprised of five to seven different classes, all racing at their own pace. It is quite a spectacle. Similarly stock car racing is popular in North and South America. There are Supercar series in Australia, Germany, Japan, and Argentina etc.
For motorcycle fans there are a number of two wheel racing options, MotoGP topping the scales of as good as it gets. From my experience cycle marshals are a different breed to car marshals, but either event is a blast to participate in.
And finally there is local club racing. Amateur drivers, semi-pro, up and coming stars, etc. I have outlined some of the series by country below. Contact your local club for more details. Notable series include F1, Indycar, Grand-Am, Trans Am, ALMS, WEC, DTM, SuperGT, V8 Supercars, V8 Supertourers, Stock Car, TC2000, MotoGP, WTCC, World Challenge.