This is my favorite presentation tool. I find it very practical and easier to use particularly with students in the classroom.Google Presentation also provides you with a store of great templates to choose from so you will always have new backgrounds to use.
I do love Prezi. It allows you to create great an engaging presentations. I use it mainly for presentations I create for Master courses.
Prezentit is another cool web tool that lets you create presentations online. It does not require any software installation and is very easy to use and also offers synchronous collaboration on slides you make with friends.
HelloSlide transforms your slides into a rich audiovisual format, recreating the experience of a live lecture. Simply upload your presentation, type the speech for each slide, and HelloSlide automagically generates the audio.