Tires.

I have $15 Kenda tires.

http://www.chainreactioncycles.com/M...?ModelID=24539

These are going on my bike. The rubber compound is better than my current tires. 700c x 35. They'll grab the wet roads better. They're not for mountain/offroad, I'm aware.

You can find whatever grippy slicks you want, but make sure you have tires that don't dump you on the ground the moment you ride over a shiny patch of damp pavement. Rubber compound makes a huge difference. No matter what kind of awesome goretex jacket/pants you have, you're still going to hit the ground when your bike skids out from under you.

Get disc brakes or Kool Stop pads or whatever someone recommends for the rain. You don't want cheap rim brake pads that glide along the rim over a bead of water; get something made of a rubber compound that cuts through water and grabs that rim tight.