Apache JMeter is an Apache project that can be used as a load testing tool for analyzing and measuring the performance of a variety of services, with a focus on web applications.
JMeter can be used as a unit test tool for JDBC database connections, FTP, LDAP, Webservices, JMS, HTTP, generic TCP connections and OS Native processes. JMeter can also be configured as a monitor, although this is typically considered an ad hoc solution in lieu of advanced monitoring solutions. It can be used for some functional testing as well.
JMeter supports variable parameterization, assertions (response validation), per thread cookies, configuration variables and a variety of reports.
JMeter architecture is based on plugins. Most of its "out of the box" features are implemented with plugins. Off-site developers can easily extend JMeter with custom plugins.