I have always liked using "Frozen Charlottes" in my mixed media work - but they're getting harder to find - and more expensive when they are available.
These are the perfect alternative. They're extremely lightweight plastic, so they don't really seem like the real deal until they're glued into a project; then, it's hard to tell the difference.
They tend to be a little too shiny - but some inking takes cares of that - my only complaint (if it's really a complaint) is that they only come in the two sizes. These are the perfect sizes for using in Tim Holtz transfiguation boxes - but a slightly larger size for other uses would be nice.