Lots of things, but one that has many specific forms is especially annoying. That's all the little things that really are little but require more attention to do than you might think and often repeat themselves even thousands of times during one's lifetime: brushing your teeth, doing the dishes, shaving, finding the right key, logging in to services that log you out periodically... It's all so easy but repetitive and possibly time-consuming as well! I hate it!
Also, people in general deserve a special mention with the little annoying and/or stupid things they do. Thanks a lot for blocking the hallway in the grocery store, good sir! It makes perfect sense to put the cart sideways right there in the center. And more seriously, it's even much lesser things that annoy me.