Server performance

Cache/Load balance

HAProxy : C : load balancer
Free, fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications
memcached : C : memory cache
Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load
Varnish : C : caching proxy
Web application accelerator also known as a caching HTTP reverse proxy

Test

bombardier : Go
Fast cross-platform HTTP benchmarking tool
WebPagetest : PHP
Free website speed test from multiple locations around the globe using real browsers (IE and Chrome) and at real consumer connection speeds
wrk : C
Modern HTTP benchmarking tool capable of generating significant load when run on a single multi-core CPU

See also

Further reading