Another option for Python users:
from jenkinsapi.jenkins import Jenkins
#get the server instance
jenkins_url = 'http://<jenkins-hostname>:<jenkins-port>/jenkins'
server = Jenkins(jenkins_url, username = '<user>', password = '<password>')
#get the installed plugins as list and print the pairs
plugins_dictionary = server.get_plugins().get_plugins_dict()
for key, value in plugins_dictionary.iteritems():
print "Plugin name: %s, version: %s" %(key, value.version)