Although its development has been discontinued some years after Commodore went under, SAS/C Version 6 still is one of the best and recommendable C-compilers for classic AmigaOS development until today.

vbcc is a highly optimizing portable and retargetable ISO C compiler by Volker Barthelmann. It supports ISO C89 and a subset of C99. It's also still being developed and offers a variety of target distributions and cross-compiling options. Best of all: It's free.