DDR4 memory has been available for some time now, but with support limited to the X99 architecture, it thus far hasn't overtaken DDR3 as the go-to memory to use. And it's still far more expensive than DDR3. But as newer Intel chipsets support DDDR4 and it becomes more ubiquitous in the coming year, it's a good time to look ahead to the memory types we might be using after DDR4, and Extremetech has a great primer on three new and upcoming memory technologies: Wide I/O, High Memory Bandwidth (HMB) and Hybrid Memory Cube (HMC).
Wide I/O (and Wide I/O 2) is a high-bandwidth, low-power system designed (and most useful) for mobile SoCs. The standard has been backed by Samsung and other smartphone manufacturers as high-res handheld displays require lots of bandwidth but using as little power as possible is critical to battery life. Wide I/O is the first version of the standard, but it's likely that Wide I/O 2 or 3 is the version that actually makes it to market. No major devices are expected to ship with Wide I/O in the first half of 2015, but late 2015 may see the standard gain some limited ground.
According to Crucial, DDDR4 bandwidth maxes out at about 25.6 GB/s. Wide I/O, on the other hand, has a bandwidth of 12.8 GB/s, at least according to an old Xbitlabs post. Wide I/O 2 or 3 may offer significantly more bandwidth, and keep in mind that this is a technology designed for power efficiency first and foremost. It could eventually be a big deal for gaming laptops and other portable hardware.
Next we have Hybrid Memory Cube (HMC), a joint standard from Intel and Micron that offers significantly more bandwidth than Wide I/O 2 but at the cost of higher power consumption and price. HMC is a forward-looking architecture designed for multi-core systems, and is expected to deliver bandwidths of up to 400GB/s, according to Intel and Micron. Production could begin next year, with HMC commercially available in 2017.
Finally, High Bandwidth Memory is a specialized application of Wide I/O 2, but explicitly designed for graphics. (Both AMD and Nvidia plan to adopt it for their next-generation GPUs.) HMB can stack up to eight 128-bit wide channels for a 1024-bit interface, allowing for total bandwidth in the 128-256GB/s range. In other words, it's not as cheap or power efficient as Wide I/O, but it should be cheaper than HMC.
Also, since it's designed explicitly for high-performance graphics situations, future GPUs built with HBM might reach 512GB/s to 1TB/s of main memory bandwidth. That's a not insignificant upgrade over the current top-end 336GB/s Titan Black.
If you enjoy digging into the nitty gritty of future memory formats (and who doesn't really?), check out Extremetech's full article for a deeper dive.