WikiTree, detailing The How and The Why. The "Why" hasn't changed: WikiTree is still great cousin bate. In fact, since writing that blog post, a unknown cousin found the page on my 3x Great-Grandfather, Richard T Smith, and sent me a photo of him!
The "How" on the other hand, has changed some. When I wrote that post, I said that although you could add your tree via Gedcom upload, I recommended against it (except in the case of a few generations at a time). The reason for this was to prevent accidental duplication of individuals in the WikiTree database. WikiTree's purpose is to create one giant family tree; if you share an ancestor with another user, you should both share access to the profile and edit it together.
I have to take that back now, as WikiTree has recently introduced a great new feature: Gedcom Comparisons. Now, when you upload your Gedcom file to WikiTree, you can automatically find matches between your tree and the WikiTree database. Your matches are presented in a table and you can compare each match in a side-by-side comparison view. If you do find matches, you can connect your tree with the ancestors already online and contact your newfound cousin!
If you've hesitated to try WikiTree before, this new feature is a great reason to try it out. For more information on getting started, check out the press release at GeneaPress.