Yandex.Tank is an extendable open source load testing instrument for advanced linux users which is especially good as a part of automated load testing suit.
Documentation at ReadTheDocs
Blog - in russian, though you're welcome to ask in english if you want
Evgeniy Mamchits' phantom - Phantom scalable IO Engine
Gregory Komissarov's firebat - test tool based on Phantom
Andrey Pohilko's loadosophia - service for storing and analysing performance test results
Jenkins - an extendable open source continuous integration server that may be used to automate test execution.
Graphite - an enterprise-scale monitoring tool, use it to store your test results and render graphs.