rules2mrtg is a tool that creates MRTG data traffic graphs derived from iptables's internal traffic statistics for the local machine for every configured IP number/ethernet alias. You can also define ports or port ranges to be monitored per IP address. This avoids problems with using SNMP and aliased interfaces.
Surealived is a effective LVS (Linux Virtual Server) service tester. It is based on epoll and allows you to detect your hosts' state using HTTP, TCP, DNS, exec, and a universal LUA embedded tester. SSL is also supported. Testers are dynamic modules, and you can extend and write your own modules. You can also dynamicaly change the weight or set real states at run-time. Test states are injected into the kernel IPVS table using the ipvssync application. Configuration is highly extensible, so many testing protocols can be implemented easily.