Here are the thumbtack balls that I made the other week. You can see where I got pinned idea from in this post. I'm debating coating them with some sort of sealer so the tacks stay put. And I still may end up painting them but overall I think they turned out pretty darn cool.
To make these I bought styrofoam balls from Michaels (so cheap) and a ton of tacks from Staples. Each ball took about 200 thumbtacks. There are pretty clear instructions linked to my previous post.