Free: http://openinviter.com/
Paid: http://www.octazen.com/ http://addressbookimport.com http://www.improsys.com/importer.htm
Most of these have the best support for PHP which may or may not work for your needs. Various free libraries have been released for other languages but 1) generally support only a few email providers and 2) may not be actively maintained. Octazen has a fee-based API which sounds great but I haven't used it.