xNBD is yet another NBD (Network Block Device) server program, which is fully compatible with the NBD client driver in the Linux kernel. In comparison with the original NBD server, it provides possibly better I/O performance by using mmap(), concurrent access from multiple clients, distributed Copy-on-Write disk, and live storage migration for virtual machines.
Perlbal is a Perl-based reverse proxy load balancer and Web server. It processes hundreds of millions of requests a day just for LiveJournal, Vox, and TypePad, and dozens of other "Web 2.0" applications. One of the defining things about Perlbal is that almost everything can be configured or reconfigured on the fly without needing to restart the software.