Probably the simplest to set up and get going. This is getting better as Google add new features but there is precious little control over how it looks and a poor selection of designs.
Long established and one of the favourites and still mostly free.
PollEverywhere is very simple to set up and has the extra feature that enables people to vote using their mobiles with the results being updated live. Looks cool on a whiteboard!
There's a range of applications out there which enable you to collect votes on things and display the results.
Great for research as people much prefer clicking on-