Alexander Podelko's Performance Testing Links

Oracle Application Testing Suite (formerly e-Test Suite from Empirix) probably is the best-known Web-only commercial tool, several good white papers (Empirix still provides solutions for VoIP, IMS, and Contact Centers - Hammer; Empirix white papers).
Alex Amat's blog

Visual Studio Team Suite and Test Edition include a load testing tool
Microsoft's Patterns & Practices Performance Testing Guidance project
Shai Raiten's blog

SOASTA CloudTest, there is CloudTest Lite, free downloadable edition
    Cloud Corner blog
    Performance Matters blog
    Fred Beringer's blog

Facilita's Forecast, claims multiple technology / protocol support

LoadStorm lowest cost cloud load testing tool
LoadStorm performance tetsing blog

RadView WebLOAD
RadView Software sponsored WebLOAD Open Source Load Generation Engine

WAPT is a load and stress testing tool for web interface, inexpensive ($350 per installation), free trial
WAPT 4.0: Web Site Load, Stress and Performance Testing

eValid, "test complex web applications from inside the browser to work from the same perspective as your users"

PushToTest TestMaker, allows to re-use Selenium, Sahi, soapUI test scripts and unit tests written in Java, Ruby, Python, and PHP for performance testing

There are many specialised load testing tools, many for Web or database testing. See, for example:
http://www.softwareqatest.com/qatweb1.html
http://testingfaqs.org/t-load.html

Here is the list of open source load testing tools:
http://www.opensourcetesting.org/performance.php

A couple of open source tools that looks more mature:

OpenSTA Originally developed by Cyrano as a commercial tool. OpenSTA stands for Open Systems Testing Architecture. Another branch of the Cyrano code is a commercial tool QTest from Quotium.
OpenSTA versus LoadRunner by Andy O’Brien and Antony Marcano
Danny Faught's review of OpenSTA in Open Testware Reviews

Apache JMeter
     Load Testing your Applications with Apache JMeter by Keld H. Hansen
     Load Testing with Apache JMeter and Badboy – A Tutorial
     JMeter Links a big collections of links from JMeter site

Eclipse Test & Performance Tools Platform Project

JUnitPerf a collection of JUnit test decorators used to measure the performance and scalability of functionality contained within existing JUnit tests

p-unit an open source framework for unit test and performance benchmark

Hardware appliances can be useful if it is necessary to test network devices or simulate a big number of users with limited scripting requirements. For example, Spirent Avalanche or Ixia products.
SwiftTest allows to test network storage devices.
BreakingPoint provides cyber security solutions.

Comparisons

An Overview of Load Test Tools by Julien Buret and Nicolas Droze (2003), includes OpenSTA, DieselTest, TestMaker, Grinder, LoadSim, JMeter, LoadTest, Rubis

Load Test Tools Evaluation by Abraham Jacob et al. (2002)

Stress, Load, Volume, Performance, Benchmark and Base Line Testing Tool Evaluation and Comparison by Cordell Vail

Load Testing and Performance Monitoring as Services

Gomez Web and mobile performance, many good white papers

Keynote testing, monitoring and measurement products and services

Webmetrics monitoring services, managed load testing, and web performance management

Site Confidence web performance monitoring and load testing services

Catchpoint Real-Time Web Performance Monitoring

Yottaa Web Performance Monitoring / Optimization

BlazeMeter, a 100% JMeter-compatible, self-service load testing cloud

Related Tools

Shunra Software products allow real network simulation during load testing. Integrated with LoadRunner.