I'm not sure what kind of fxnality you're looking for, but
Ancestry.com is the authoritative online source for genealogy. It's run by the The Church of Jesus Christ of Latter-day Saints. As is
Familysearch.org. The nice thing about Ancestry, though, is that it integrates with
MyFamily.com for creating family trees, sharing data with other family members, etc. They are also the company that drove my old company out of business.
EDIT: Upon researching FamilySearch.org, I found that their search is free, which I didn't realize, and their help section mentions the following:
The information will be downloaded in GEDCOM format, which can be used with Personal Ancestral File and other family history record-keeping software.
I'm downloading Personal Ancestral File right now to check it out.
- trs