Kamailio, formerly known as OpenSER, is an open source SIP server, named after a Hawaiian word meaning talk, to converse. Kamailio provides complimentary SIP services to any SIP stack. It's not uncommon for Asterisk, FreeSWITCH, and other SIP media servers to be fronted by Kamailio when scaling those SIP stacks.
Kamailio is ideal for large-scale telephony deployments. It's flexible, scalable, and has dozens of impressive server features that make it a strong option for handling thousands of calls per second, as well as operating efficiently with limited resources.
At Lintel, we enjoy using Kamailio and we're always impressed with its rapid development cycles. We have high hopes for Kamailio's future.