Is there such a thing as a spill proof sippy cup?  

Well, there is no such thing as a spill proof cup, unless its empty. Even if you bolt the lid on and turn it upside down the air bubbles will rise to the top and push liquid out the drinking hole. We did find one from playtex that had a screw on lid, I just made sure not to put any dark liquids in it just in case. Also, make sure that the divider (plastic thingy) is clean and fits snug in the hole that allows juice to come through, the child has to garner a good amount of pull to get the juice out and if there are any reasons that liquid can get through it will without help. Hope that helps.

I found some disposable ones at Target called Take and Toss. They are reusable, but are inexpensive enough to not worry about losing or throwing away if you are out somewhere and don't want to carry an empty, dirty cup with you. They don't leak from the spout when they are new, but my son sometimes chews on them and after a while they start to leak. They are great, but the tops come off easily, so if he drops it I have to check the top, it comes loose and then will come right off and dump the whole cup. They're not perfect but the best I have found.

I did use a Nuby cup when he was younger. It didn't leak, but it was very hard to drink from. He hated it, so I tried it and found it was not easy even for me.

What I have started doing is just make sure that he has milk at the table, since I don't want stinky milk all over, and don't give him juices that will stain. He is also not allowed to bring his cup certain places where I am not willing to risk a spill, like in the bedroom. If he is drinking water the spills don't bother me.

I am interested in seeing the replies to this discussion because I was just looking at sippy cups in the stores recently and it seemed to me they were all made wrong. Maybe I should have been a sippy cup designer. I wonder, too, if there is a slurpy cup for kids that are "too big" for sippy cups.

Well they say that there is but I say there is not. I have two kids and I tried them when they were young and they never worked. It always made me so mad because there were times when I spent some money on them trying to get one that wouldn't spill but it seemed no matter how much I spent they still never worked and would spill juice all down the front of their shirts and made them look absolutely filthy. And it always seemed to spill right after you left the house and couldn't change their clothes anymore.
It eventually got to the point where I had to teach them to drink from a real cup sooner than normal because I was tired of wasting my money and ruining clothes.

I got these ones at Walgreens for my daughter before that worked really well and only $1 a piece. They are like the Nuby brand, but the valve is different. Those never leaked on me. Playtex ones are good, but after awhile you have to replace the valves or they do start to leak.

Go for good brands you get your cup. If good brands also give same result then you shoud need to satisfy on that result and adjust youself according to brands.

My son has used almost nothing but Avent bottles and sippy cups. I LOVE them!! I found it to be a great system because all the sippy cup tops and bottle tops fit on both the bottles and sippy cups. I never had a problem with them spilling. There is a bit of leakage when he hasn't got all the fluid out of the valve, but we are talking about 2 or 3 drops. They have a soft spout that's like a nipple, but flattened out ( they say it's for 6 month+) and a hard, plastic one (they say 18 month+).

Over the last few months I have bought other sippy cups, but none I liked as well. We had bought some for a dollar store, they have Diego, Dora or Sponge Bob, that are really cute, but there is no valve or anything. They are great for at Grandparent's houses where you don't use them everyday and don't need a huge amount. I have also bought similar ones to the take and toss, but I think they were a different brand. But I agree with the other mom that said when they are dropped the lid comes off, happened to me MORE then once. My son also chewed them causing the spout to crack, my niece did the same thing. With those you also have to watch that they have holes in them. The holes will be marked, but might not be all the way through. My sister ended up with one of those, and so did I. All those cups did was make our children VERY angry.