Rit dye is one of those great craft products, like Mod Podge and Aleene's Tacky Glue, that is classic, cheap and multi-functional, and has awesome packaging that doesn't look like it has changed since the '70s.
Of course I've used Rit to dye fabric and yarn (and yes, even a tie-dyed shirt and over-dyed vintage slips back in the day), but it's fun to come up with other things to apply color to that aren't necessarily fiber-based. Back when I was working on Handmade Modern with Todd, we used Rit to stain some pieces of cork for a nightstand, which turned out great. And I've recently been playing around with staining pegboard and wood with it too. What else could we use this amazing dye for? A quick search on Flickr showed a few Blythe dolls with Rit-dyed hair, and the Rit website suggests using it to dye paper as well. The possibilities are endless!