Well there's GForth for a start - a standard package in Debian - many others, I'm sure.
And not a Pi, but cheaper is the Fignition board - amazing. I saw it being demoed by its creator at the weekend - outputs to composite video done entirely in software on an 8-bit Amel microcontroller.
ok - after sudo apt-get install gforth:gordon @ dot: gforth
Gforth 0.7.0, Copyright (C) 1995-2008 Free Software Foundation, Inc.
Gforth comes with ABSOLUTELY NO WARRANTY; for details type `license'
Type `bye' to exit
1 2 3 * + . 7 ok
: hi ." Hello, world" ; ok
hi Hello, world ok
So there you are - just remember, its running bog-standard (Debian) Linux, so if there's a package for it, then it's almost certian it'll run on the Raspberry Pi. (Although it's not a dedicated Forth "distro" as such)