Teac briefly sold a standard tape cassette that opened to allow the listener to swap out its tiny little reels. This made for an ultracompact music collection–dozens of albums in a pocketable pouch–but it didn't take off.
A look at an unusual cross between an open reel tape and the compact cassette. As far as I know the O'Casse was only available in Japan. – you can usually them on ebay http://goo.gl/XW6kx8 (at great expense) alongside the brilliant Sony WM-D6C Walkman
It's interesting just how many ways they tried to get performance and professional-grade utility out of this trash-ass media format and I'm so glad it's back in fashion.