Monitor your haproxy cluster using zenoss and snmp
You'll need to configure haproxy to use socket for stats[1], and your snmpd to run a perl script[2] to read from that socket (trust me this is pretty easy). You'll also likely need to update the PythonCollector ZenPack and install the CalculatedPerformance ZenPack.
[1] http://serverfault.com/questions/273820/haproxy-perl-plugin-and-net-snmpd-on-ubuntu-10-04
[2] http://www.haproxy.org/download/contrib/netsnmp-perl/haproxy.pl
note: this is adapted for the community from another proprietary version I had written. I have not actually tested this in production. but... it should work. Please test and submit issues or pull requests.
This has been tested on 4.x, 5.x, and 6.x but should even work on 3.x.