HAProxy is known to reliably run on the following OS/Platforms :
Linux 2.4 on x86, x86_64, Alpha, Sparc, MIPS, PARISC
Linux 2.6-5.x on x86, x86_64, ARM, AARCH64, MIPS, Sparc, PPC64
Solaris 8/9 on UltraSPARC 2 and 3
Solaris 10 on Opteron and UltraSPARC
FreeBSD 4.10 - current on x86
OpenBSD 3.1 to -current on i386, amd64, macppc, alpha, sparc64 and VAX (check the ports)
AIX 5.1 - 5.3 on Power™ architecture