Does OS X have an equivalent to /dev/shm?

You can create and mount a ram based disk as follows:

1. Create a disk

   hdiutil attach -nomount ram://$((2 * 1024 * SIZE_IN_MB))
   

   hdiutil will return the name of the ramdisk.

2. Format and mount the disk

   diskutil eraseVolume HFS+ RAMDisk NAME_OF_DISK
   

3. Access the disk under /Volumes/<diskname>

   cd /Volumes/RAMDisk && touch testfile.txt && ls
   

Creating a 100MB ramdisk:

$ hdiutil attach -nomount ram://$((2 * 1024 * 100))
/dev/disk3

$ diskutil eraseVolume HFS+ RAMDisk /dev/disk3
Started erase on disk3
Unmounting disk
Erasing
Initialized /dev/rdisk3 as a 100 MB case-insensitive HFS Plus volume
Mounting disk
Finished erase on disk3 RAMDisk

Original source: http://www.frederico-araujo.com/2008/12/18/blazing-fast-firefox-using-osx-ramdisk/