Well, my roommate of 13 years have been registered as domestic partners for several years. We did this when we found out that I could tag onto his company paid insurance, since I was not yet eligible for Medicare.
A California domestic partnership is a legal relationship available to same-sex couples, and to certain opposite-sex couples in which at least one party is at least 62 years of age. It affords the couple most but not all of "the same rights, protections, and benefits, and shall be subject to the same responsibilities, obligations, and duties under law..." as married spouses.
Enacted in 1999, the domestic partnership registry was the first of its kind in the United States created by a legislature without court intervention. Initially, domestic partnerships enjoyed very few privileges—principally just hospital-visitation rights and the right to be claimed as a next of kin of the estate of a deceased partner. The legislature has since expanded the scope of California domestic partnerships, though these still do not provide all of the rights and responsibilities common to marriage. As such, it is now difficult to distinguish California domestic partnerships from civil unions offered in a handful of other states. UnQuote