This folder contain a number of external libraries.

Sourced from $TEAMCITY_HOME/devPackage/
agent-api.jar
common-api.jar
runtime-util.jar
server-api.jar

Sourced from $TEAMCITY_HOME/webapps/ROOT/WEB-INF/lib/
annotations.jar
log4j-1.2.12.jar
openapi.jar
patches-impl.jar
resources_en.jar
trove4.jar
util.jar

Sourced from AccuRev:
AccuRevCommon.jar