Sas/C is a great 68k compiler but a little
hard to find now. If you are using UAE
(since you asked about an adf) or a slightly
upgraded machine you could probably use VBCC.
VBCC is newer, freeware and a good cross-compiler.
GCC is decent too but will take extra effort
to install and setup. You may have to obtain
the AmigaOS include files seperately from the
compiler, they are included in the Amiga NDK
(Native developer's kit) which has been
available online at various times or on CD.