The Public Knowledge Project has released Open Archives Harvester, an open source OAI-PMH harvester.
Here's an excerpt from the announcement:
This is a major rewrite of numerous parts of the Harvester code, including metadata storage and indexing. It increases indexing flexibility to support plugin-based indexing, including Lucene/SOLR support. It also adds OAI Data Provider support, including the potential to convert between metadata formats (currently from various formats into Dublin Core).