I've fallen in love with Socrative. It's a great tool, especially in a 1:1 environment. Essentially, it replaces the clickers that schools used to (still?) spend a ton of money on.
I create my class - and I can name my room anything I like (go to Profile->Change Room Number and you can type words too). It's great, my students know how to find me, and no login is required.
Students log into my room and I can fire questions anytime I like - I tend to make them up as I go, or I build them into my slides. It's very quick - press a button, they get a multiple choice or true/false prompt, and off we go.
It's not supposed to be for summative assessment - I can make strong arguments against multiple choice (or tests in general) as an assessment tool. What it is excellent for is the ability to quickly assess where my learners are - "did they get it?".
"The crest of the wave is a), b), c), d)". Bam - 20 seconds and I know if they understood the diagram I just put up :)
The challenge comes in deciding what number is okay to move on - in my class of 11 Physics students, for example, anything less than 9 right and I'll go back. But what about those two who didn't get it, and I moved on? I struggle with how to best support them - I try and go back after the lecture portion, during an activity or homework time, but don't always remember the exact issues they had.
Thus, while I love the tool, I'm going to have to figure out how to not leave anyone behind.