Several plugin bundled with TeamCity are developed by JetBrains and are open-sourced under Apache 2.0 license.
The plugins include:
Eclipse Integration
ClearCase VCS Support
FxCop Runner
Rake Runner
REST API
(bundled since 5.0)
documentation
XML Test Reporting
(bundled since 4.5)
documentation
Mercurial VCS Support
(bundled since 5.0)
plugin page
GIT VCS Support
(bundled since 5.0)
plugin page
SourceGear Vault
(bundled since TeamCity 5.1)
plugin page
Build Queue Priorities
(bundled since TeamCity 6.0)
plugin page
Swabra
(bundled since TeamCity 5.1)
plugin page
NuGet
(bundled since TeamCity 7.0)
plugin page
Agent Info
(bundled since TeamCity 7.0)
svn root: http://svn.jetbrains.org/teamcity/plugins/agent-info/trunk
PowerShell Runner
(bundled since TeamCity 6.5)
plugin page