Other front-ends are available outside of gcc's SVN repository:
Old front-ends which were removed for one reason or another:
Chill (no-one interested to integrate it in gcc3)
There's also a mythical creature, a PL8 front-end developed by IBM to design the zSeries (s390) processors. PL8 is the language used for the firmware in the zSeries processor; and because the microcoded instructions in the newest zSeries actually execute non-microcoded instructions from the very same processor, IBM compiled these microcoded instructions using a PL8 front-end together with the existing s390 backend.
Information about writing new front-ends can be found on WritingANewFrontEnd.