Slip Cups are a set of plastic cups ($5 for 20) that sit inside the regular 16 and 18-ounce plastic cups used for beer pong. They fit snugly inside the cups above the beer-line and are not filled with beer themselves, preventing players from having to drink the hair and dirt that typically covers a beer pong ball. They also have two flaps on the inside that capture balls that enter and prevent them from bouncing out (although this also prevents playing by beer pong rules that allow blowing). Personally, I thought drinking cups of beer with all kinds of disgusting shit in them was part of the game. That's why you want to win as quick as possible, so you don't have to drink so much shit off the floor. Besides, you're at a college house party, not a fancy restaurant -- are you really that concerned about a little hair in the Natty Light?
Keep going for a shot of the cup's ball-catching flaps and their Kickstarter video.
Thanks to Douglas, who called the invention the best thing to happen since sliced bread provided you are unaware of any other invention that's come after sliced bread.