Vynil, cassettes, all that, they were a long time ago; nowadays, blu-ray disks rule ! But it seems we've been using them for quite a while, so one can only wonder what's the next step in optical disk technology ? Well, the next step is called HVD, which stands for Holographic Versatile Disk, which employs a technology called collinear holography.
In collinear holography, a green and a red laser are collimated into a single beam; the green laser data encoded as laser interference fringes from a holographic layer near the top of the disc, while the red laser serves as a reference beam and reads the servo invo from the aluminum layer, pretty much like in normal CDs.
The laser beams pass selectively through the layers due to a layer of dichroic mirrors that exists between the holographic and servo data layer. The HVD will be able to hold about 20-200 times more information than a blu-ray disk, and a transfer rate of 128 MB/second.
However, they are incompatible with current technology, and probably won't appear for another 5-10 years; most plans are for implementing them in 2019 or 2010. However, it's always good to know what the future holds for you... especially when it holds so much.