Network performance is only one part of a distrbuted system's perfomance. An equally important part is the performance od the actual application and hardware it runs on.
Web100 - fine-grained instrumentation of TCP implementation, currently available for the Linux kernel.
SIFTR - TCP instrumentation similar to Web100, available for *BSD
DTrace - Dynamic tracing facility available in Solaris and *BSD
NetLogger - a framework for instrumentation and log collection from various components of networked applications