I am not a fan of 1990s-looking presentations full of childish Microsoft clipart, but this site Open Clip Art can still be useful resource for presentation designers. Firstly, clip art designers are moving into full illustration designs. Secondly, the site is a good place to find icons for your presentation. All material is free to use. As with all user generated content sites, there is no professional reviews before designs get posted, you need to select the poor from the great yourself.
SlideMagic: a platform for magical presentations. Free student plan available.