There are already a few other data portals, and some of them (unlike Socrata) are Open Source.

CKAN - http://ckan.org

OpenDataCatalog - https://github.com/azavea/Open-Data-Catalog/

Junar - http://www.junar.com/

DKAN - http://nucivic.com/dkan/

PublishMyData - http://www.swirrl.com/publishmydata

CKAN in particular is used by a lot of National Governments ( http://ckan.org/instances/ ) , is Open Source, has a reasonably large developer community. It also isn't quite as expensive as some of the closed source commercial offerings.

There are a few more but in various stages of completion/usefulness.

Disclaimer: CKAN dev.