Students brainstorm a topic and use the letters of the alphabet to list subtopics or related words. For example, I use this in my Geography and World Cultures class and if I give the topic of "the Dominican Republic" students could list:
A- artesian work
S- salsa music
T - Taino indians
This can be used as a race or a competition between individuals or small groups. I usually then discuss it as a whole class and write a master list of words on the board. Enjoy!