Magento Mass Importer (MAGMI) is an open source project to improve on the infamously poor speeds of Magento's default import tools.
The Magento Mass Importer (MAGMI) project's initial goal was to provide a product mass import script from CSV files that match the Magento dataflow CSV export format.
As explained in Project Origins page, the Magento dataflow was not an option when dealing with several thousands product catalogs.
The main project goal is to achieve dataflow like operations with a reasonable speed.
There are also various plugins available for the MAGMI script which allow for on-the-fly operations, including manipulation of the product data and creation of categories. Details at the Magmi Wiki.
For latest changes visit Github page.