I spent a few minutes testing out the features of socrative they seem to be well thought out and worked just fine on my desktop and iPod.
Cheap as in free
Easily add impromptu questions.
Easy for students to login and learn.
Both asynchronous and synchronous quizzes.
Visually simple design
Not sure how long the website will be around.
You have to enter all of the questions.
Great website that I am looking forward to using in my classroom.