Enabling High Performance Data Transfers - System Specific Notes for System Administrators (and Privileged Users), Web page edited by Matt Mathis and Raghu Reddy (PSC). Covers performance basics and includes tuning hints for many Unix variants as well as Windows.
TCP Tuning Guide, by Brian L. Tierney (LBNL). A short guide on TCP buffer tuning, with specific sections on Linux, FreeBSD, and Solaris.
Unix IP Stack Tuning Guide by Rob Thomas (Team Cymru). Slightly dated but compact and covering many Unix variants.