Varnish is a HTTP accelerator, build from ground up with performance in mind. Varnish is targeted primarily at the FreeBSD 6/7 and Linux 2.6 platforms, and will take full advantage of the virtual memory system and advanced I/O features offered by these operating systems. You can get more information and the software from Varnish site.
http://varnish.projects.linpro.no/
You can monitor Varnish statistics using varnishstat and varnishtop command.
http://www.ibm.com/developerworks/opensource/library/os-php-varnish/