I started a Reddit thread on finding an experimental filesystem, that used RAM to speed up drive access. However, there isn't a clear-cut use of this in anything since it was proposed ~10 years ago.
* SuperSpeed hawks some patented drivers and software for accelerating drive IO in Windows. Most of their product line is geared towards pre-2008.
* Lookup "cache file system" or "CacheFS" in regards to methods on Unix & Linux to accelerate use of network or optical drives via HDD cache (I assume a RAM-drive could be used with this).
* Windows Vista / 2008 and later auto-cache files into available memory: its rather sloppy though, and had to be fine-tuned in 7 / 2008R2. There's a programmable service the Windows Debugger team put together to apply this tuning to pre-7 versions of Windows: Dynamic Caching Service