Synthetic-bristle paintbrushes are useful for trim and other tight spaces, but only if you're using matte paint. Glossy paint shows brush stokes more easily, so use foam brushes to cut down on visible brush marks if you are using glossy paint.
Medium-nap rollers work for matte finishes, but foam rollers are better suited for glossy finishes. Many hardware stores also sell special textured rollers to create faux finishes and sponging effects.
Painter's masking tape shields trim and fixtures from paint splatter, and it peels off without leaving behind sticky residue. Plastic sheets and canvas drop cloths are great for floor protection. And protect your clothes with a smock, or wear old clothes that you don't care if you get paint-splattered.
Although you don't need them for every job, ladders and extension poles are good specialty gear to have for ceilings or vaulted walls. You also might need sandpaper, plaster and primer to prep some surfaces.