Commodore 64 RAM Disk. I remember having 64k of RAM soldered on a generic bread board with a 9V battery as backup. It made the computer ridiculously fast when you had slow floppy drives for I/O. I also made it work for the Amega but the payoff was much lower and I stopped using it because the floppy drive and hard drives were faster and the bottle neck was at the CPU.

Video Production - If I could store my video projects in a RAM Disk it really would be pretty amazing. The issue is SSD have really been able to speed up the input so much that the benefit would be much smaller. In video your write to disk is actually very small and usually only happens while you import, preview or render (Which is faster on GPU RAM). There is a lot of work to get a production RAM Disk up and being ready with a low benefit. It still would be nice to have all my clips just reside in Ram and able to "real time" render with less stutters on previews.

Data "Science" - I most use R and that is 100% in memory so it certainly has been a boom for that in my own usage. 64 GB of RAM is pretty cheap and then I can also use Spark if it isn't enough. My current biggest time restraint for big data sets is getting the sets more then anything else. Having large data sets reside in a RAM Disk would really speed things up but we now have Spark for the actual big data sets.