RXDSK is a project currently in development which is worth checking out and keeping an eye on.
It is quiet similar to ZRAM (previously compache) and TMPFS/RAMFS but has some minor differences which renders it more flexible for generic usage.
ZXDSK allows for example an export of the ram disk to a physical disk, making it therefore usable as a swap device. According to the developer, LZO is planned to be used as a compression layer, making it therefore quiet similar in this light to ZRAM. However as opposed to the latter, RXDSK acts as a sort of sparse device, since the defined device size is allocated as the filesystem is growing and not statically as with ZRAM.
To get more information, visit http://wiki.petroskoutoupis.com/index.php5?title=RapidDisk - till then, RXDSK can easily become the defacto ram device choice for RRD base collectors such as Ganglia, Cacti - offloading mysql cache to RXDSK with a persistent enabled sync storage on an SSD drive etc.