Lots and lots of threads around on this...

Helmet
Gloves
Shorts
Small seat bag
Multi-tool
Spare tube(s)
Patch kit
Tire lever(s)
Water bottle(s) & cage(s)
Floor pump
Frame pump
CO2 carts if that's your bent

As far as bottles go, I used a Polar for years and thought it was great, until the valve broke off in a spin class a few weeks ago and I replaced it with a Camelback Podium Chill. Much better valve and a lot better at keeping liquid cold, IMO.

And as in all other pursuits, there are lots of other things you can get depending on your budget and desired level of bike-geekiness.