I've tried the Advent Sippy Cup, Munchin Spill-Proof Cup, The Munchin Staw Cup (too hard for kids to suck), and a host of other no-name cups. All of them leak or are hard for babies to use.
My friend Dawn found what I will call the Miracle Cup (enter in choir music). It's the CamelBak Kids Bottle ($14 at REI). The valve makes it easy for kids to bite and drink water and in no time they figure out how to use the straw. I love how it flips down for easy tote-around. I think the age they recommend is 3 years old but I've been using this for 2 months now (since Chloe was 10 months old) and it's fantastic. I recommend this to any new mom so they can save money from all my trials and tribulations.
Update 12/01: These cups are BPA free.