The majority of my presentations have either a pitch black or bright white background. But now that monitors are getting Retina-like resolutions, it becomes possible to add a tiny, tiny texture to the background. Here is a site that has a few candidates: subtlepatterns.com. (Update 7 April 2017: new link https://www.toptal.com/designers/subtlepatterns/
On the average crappy VGA office overhead projector this effect will not come out though.
SlideMagic: a platform for magical presentations. Free student plan available.