Just came across http://www.opensourcetesting.org/ which list a whole bunch of FOSS tools used for testing. Here's the list. My favorites are JMeter and Selenium, and both came up in context of some OLPC systems at a meeting today. Selenium looks promising. We use JMeter for testing Moodle on campus. Here's the full list:

• Abbot Java GUI Test Framework
• Anteater
• Apodora
• Arbiter
• Autonet
• AutoTestFlash
• Avignon
• Blerby Test Runner
• Canoo WebTest
• Concordion
• Crosscheck
• csvdiff
• CubicTest
• Cucumber
• DBFeeder
• DbFit
• DejaGnu
• Dogtail
• Doit: Simple Web Application Testing
• Eclipse TPTP
• EMOS Framework
• Enterprise Web Test
• Expect
• Frankenstein
• GITAK
• GNU/Linux Desktop Testing Project
• Harness
• httest
• IdMUnit
• Imprimatur
• ItIN - Infopath testing in .Net
• ITP
• ItsNat, Natural AJAX
• ivalidator
• Jacobie
• Jameleon
• jDiffChaser
• Jemmy
• JFunc: JUnit Functional Testing Extension
• JSystem
• jWebUnit
• Latka
• Linux Test Project
• LogiTest
• LReport
• MActor
• Marathon
• MaxQ
• miTester for SIP
• Mockito
• MozUnit
• OLVER - Open Linux VERification
• Om
• Om
• org.tigris.mbt
• PAMIE
• Pounder
• pywinauto
• QAT (Quality Assurance Tests)
• QMTest
• Rasta
• Robot Framework
• Sahi
• Samie
• Sauce Labs: cloud-based Selenium service
• Scalable Test Platform
• Selenium
• SharpRobo
• soapui
• Software Automation Framework Support
• Software Testing Automation Framework (STAF)
• Solex
• SWAT (Simple Web Automation Toolkit)
• SWTBot
• Systin
• tclwebtest
• TestGen4Web
• TestPlan
• TextTest
• Tomato
• Toster - The Object-oriented Sofware Testing Environment
• Watij
• WatiN
• Watir
• Web Form Flooder
• WebDriver
• WebInject
• WebTst
• WET
• Win32::IEAutomation
• XML Test Suite