Family Search Indexing is a special project by the LDS Church in Utah (same folks who run Family Search). It allows you to help index records so that the Church can make the records searchable on the internet. This is a great way for you to give back and help others with their genealogical research.
I've indexed a few random census records, but the recently added Freedmen's Bureau Letters peaked my interest. While I've not found any of my ancestors in the Freedmen's Bureau records, you might find some of yours and help get records on the 'net faster.
Volunteer at familysearchindexing.org (it's free).