Lauren, this post is for you--
My favorite sippy cup by far is the Playtex The Insulator Sport Straw Cup. It has many of the same features as the Rubbermaid one that Judy was raving about but it keeps drinks cold too. Caitlin chews the crud out of the plastic straw and it has still held up. The plastic attachment straw (the one that reaches down into the glass) detaches from the cup to be washed separately. We love these cups on hot summer days. Just pop a couple of ice cubes in with the drink and it stays cool for a long time. Just don't leave it in the car!
For the non-insulated type, I recommend the Sparklin' Sipster Spill-Proof Cup. I think they come in various sizes from 6-9 oz. These have handy handles too, which are nice for the little ones to tote around. Also hangs onto your pant pocket when both hands are full! *wink*