For riding in the rain ... something I seem to do a lot of ...
Jacket - look for something waterproof-breathable, with long underarm zippers, a back vent, and in a bright color
Helmet cover - with a flap at the back to prevent water from running down your neck, and a bill in front to prevent water from running into your face
Neoprene gloves, or waterproof mitts (shells)
Neoprene booties or nylon booties (the neoprene will keep you drier, but the nylon are easier to carry)
Rain pants ... I'm still looking for those myself
And plastic bags by the hundreds to wrap everything you carry with you.