sippy cups
Some people skip sippy cups entirely (see RIE), but we used them starting around 8 months, and we fully phased out bottles by 10 months. Most sippy cups have real drawbacks, but below are my top picks. If you are interested in non-plastic and non-toxic cups, this Gimme the Good Stuff Sippy Cup Guide is very comprehensive.
zoli baby bot
When we first transitioned from a bottle to a sippy cup, the Zoli Baby Bot was great because it has a narrow straw that prevents too much gulping. It also has a weight at the end of the straw, so the cup and straw function even if the baby holds it upside down. That said, there are some real downsides: this cup is prone to leaking and it's pretty annoying to wash all the tiny parts. The Zoli cup works well if you are preparing a bottle/feeding for immediate consumption, but it's not the best for an outing because there's a good chance it'll leak all over your bag. Our on-the-go solution was to put the milk in a S'well bottle to keep it cold and contained, and then pour it into the Zoli cup only when it was time to drink.
thermos foogo
The Thermos Foogo has a wider straw than the Zoli so we didn't use it for the transition from bottle to cup, but introduced it a bit later. It does an incredible job of insulating and keeping cold, and the only drawbacks are that it is prone to some leaking (less than the Zoli, but not inconsequential) and it has lots of parts that are annoying to clean (little pipe cleaners for the straw, for example). Also, be careful if you open this on a plane: we've had projectile situations where milk went everywhere!
munchkin miracle 360
These are my absolute favorite. It's like drinking from a normal cup, except without the spills. It's a little hard to explain how the Munchkin Miracle cup works, but here goes: the colorful top silicone piece snaps snuggly into the white plastic, covering lots of tiny holes on the white plastic. When the baby sips from the cup, the pressure lifts the silicone only where the baby's mouth is making contact, and the beverage is released. It's basically magic. Also, this cup consists of only three pieces and everything can be cleaned in the dishwasher. It's the best. The only downside is the lack of insulation.
universal sippy cup lids
Boon makes stretchy silicone sippy cup lids that can be used almost universally to fit over a regular cup or glass. They also make a version with a straw top. These are extremely convenient when eating out, and also really easy to clean.