snmp-server enable traps snmp authentication linkdown linkup coldstart warmstart
snmp-server host IP.OF.TRAP.HOST COMMUNITY_STRING_HERE snmp
Though since you are doing this you should consider SNMPv3 where available -- it's less important for traps than for access, but still it is better to encrypt.
On the other side there are a ton of options from various NMS solutions, to simple SNMP-to-syslog trap translators syslogng/snmptrapd.
