OS X LaunchDaemon for a Jenkins Java Web Start slave. This is now maintained at https://github.com/rhwood/jenkins-slave-osx Please refer to that version instead.

install.sh

#!/bin/sh
# create jenkins group
NEXT_GID=$((`dscl /Local/Default list /Groups gid | awk '{ print $2 }' | sort -n | grep -v ^[5-9] | tail -n1` + 1))
sudo dscl /Local/Default create /Groups/jenkins
sudo dscl /Local/Default create /Groups/jenkins PrimaryGroupID $NEXT_GID
sudo dscl /Local/Default create /Groups/jenkins Password \*
sudo dscl /Local/Default create /Groups/jenkins RealName 'Jenkins Node Service'
# create jenkins user
NEXT_UID=$((`dscl /Local/Default list /Users uid | awk '{ print $2 }' | sort -n | grep -v ^[5-9] | tail -n1` + 1))
sudo dscl /Local/Default create /Users/jenkins
sudo dscl /Local/Default create /Users/jenkins UniqueID $NEXT_UID
sudo dscl /Local/Default create /Users/jenkins PrimaryGroupID $NEXT_GID
sudo dscl /Local/Default create /Users/jenkins UserShell /usr/bin/false
sudo dscl /Local/Default create /Users/jenkins NFSHomeDirectory /var/lib/jenkins
sudo dscl /Local/Default create /Users/jenkins Password \*
sudo dseditgroup -o edit -a jenkins -t user jenkins
# download the LaunchDaemon
sudo curl --url https://raw.github.com/gist/4136130/d3c9d7275ce78e050d8594037a2d509652a766e5/org.jenkins-ci.slave.jnlp.plist -o /Library/LaunchDaemons/org.jenkins-ci.slave.jnlp.plist
# create the jenkins home dir
sudo mkdir /var/lib/jenkins
# download the jenkins JNLP slave script
sudo curl --url https://raw.github.com/gist/4136130/2553c4cec9bc7ed5557359a22c6c1b61028afa40/slave.jnlp.sh -o /var/lib/jenkins/slave.jnlp.sh
sudo chown -R jenkins:wheel /var/lib/jenkins
sudo chmod 755 /var/lib/jenkins/slave.jnlp.sh
# create a logging space
sudo mkdir /var/log/jenkins
sudo chown jenkins:wheel /var/log/jenkins
# remember to edit the LaunchDaemon
echo "IMPORTANT:"
echo "edit /Library/LaunchDaemons/org.jenkins-ci.slave.jnlp.plist to use the correct master"

org.jenkins-ci.slave.jnlp.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-/Apple/DTD PLIST 1.0/EN" "http:/www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>org.jenkins-ci.slave.jnlp</string>
	<key>ProgramArguments</key>
	<array>
		<string>/var/lib/jenkins/slave.jnlp.sh</string>
		<string>--master=http://jenkins.example.com</string>
	</array>
	<key>KeepAlive</key>
	<true/>
	<key>RunAtLoad</key>
	<true/>
	<key>UserName</key>
	<string>jenkins</string>
	<key>WorkingDirectory</key>
	<string>/var/lib/jenkins</string>
	<key>StandardInPath</key>
	<string>/dev/null</string>
	<key>StandardErrorPath</key>
	<string>/var/log/jenkins/jenkins.log</string>
	<key>StandardOutPath</key>
	<string>/var/log/jenkins/jenkins.log</string>
</dict>
</plist>

slave.jnlp.sh

#!/bin/bash

JENKINS_SLAVE=`hostname -s | tr '[:upper:]' '[:lower:]'`
JENKINS_MASTER=http://jenkins
JENKINS_PORT=''

if [ -f /etc/jenkins.node.conf ]; then
    source /etc/jenkins.node.conf
fi

while [ $# -gt 0 ]; do
        case $1 in
                --node=*)
                        JENKINS_SLAVE=${1#*=}
                        ;;
                --master=*)
                        JENKINS_MASTER=${1#*=}
                        ;;
                --jnlp-port=*)
                        JENKINS_PORT=":${1#*=}"
                        ;;
        esac
        shift
done

echo "Getting slave.jar from ${JENKINS_MASTER}"
RESULT=-1
while [ $RESULT -ne 0 ]; do
        curl --url ${JENKINS_MASTER}/jnlpJars/slave.jar -o /var/lib/jenkins/slave.jar
        RESULT=$?
        if [ $RESULT -ne 0 ]; then
                sleep 10
        fi
done
JENKINS_JNLP_URL=${JENKINS_MASTER}${JENKINS_PORT}/computer/${JENKINS_SLAVE}/slave-agent.jnlp
echo "Launching slave process at ${JENKINS_JNLP_URL}"
java -jar /var/lib/jenkins/slave.jar -jnlpUrl ${JENKINS_JNLP_URL}

I am now maintaining a more complete version of this work at https://github.com/rhwood/jenkins-slave-osx Please use that version instead.