Classroom DIY and thought it was so clever that I wanted to pass it along. When a kid puts a card in at the top, it flips over inside and comes out the bottom upside-down, revealing the answer. So, if you wanted to use this with Task Cards at a center, you could just write the answer on the back. If your cards are laminated, you could use a dry erase marker so that you could just wipe off the answer when you are done with the activity. This activity was posted by Julie at Make, Take & Teach (and used with her permission here). Check out her terrific video tutorial on how to make it.
If you have cards that are printed four to a page, this flip chute won't work because the cards will be too big. One thing you could do is reduce the size before you print. I could see this working really well with younger students.