My dog just drank soap water that I left outside the house. I don't know what to do.
Will this negatively affect his health? Should I take him to see a vet?
It basically does the same thing to dogs that it does to humans, which means he's in for a bit of an upset tummy, a whole lot of gas, and very likely uncontrollable bowel movements depending on how much he drank and how soapy the water was. It should pass fairly quickly, but if it doesn't you should definitely contact a vet.
Also, read the label to see if there's anything in that soap but soap that might be toxic - if the label advises you to contact a poison control center if ingested, then you should definitely contact your vet. If it's household dish soap or similar, you probably don't have much to worry about - but do check.
Just make sure he's got plenty of water and make sure he's got enough water (he can dehydrate due to a very active bowel, just like humans can). It's also probably a good idea to keep him in a comfortable shady spot outside for a while for ermmm .. environmental reasons :)
Note - dogs have a pretty good sense on what they can and can't digest (though it's not perfect by far). If there was enough soap in the bin to really make him sick, he probably would not have drank it, or at least stopped after the first taste of it. Just keep an eye on him, he'll be fine.