tmpfs for FreeBSD?

[adp]

I'm looking for a ramdisk-style filesystem for FreeBSD that can be used for
scratch space, e.g., tmpfs in Solaris. The filesystem should be able to grow
and shrink in memory (and use real disk space as needed) depending on the
amount of free RAM on the system. I don't want just a fixed sized block of
memory reserved for /tmp. I will be using this for scratch files that are
quickly created and then destroyed, and will average around 2MB each. We are
expecting out tmp filesystem to need around 256MB to 512MB on average.