Another option, which may especially be suitable for ArcGIS users, is Esri Geoportal Server (not to be confused with Portal for ArcGIS.)
Esri Geoportal Server is not a part of the ArcGIS platform, and is a stand-alone application:
Esri Geoportal Server is a free, open source product that enables discovery and use of geospatial resources including datasets, rasters, and Web services. It helps organizations manage and publish metadata for their geospatial resources to let users discover and connect to those resources
One advantage for existing ArcGIS users is the ArcCatalog publish client, which allows existing metadata, which was created in ArcCatalog, to be pushed directly to the Geoportal Server.
The Open Data Catalog at http://www.opengeocode.org/opendata/ (I am a co-founder of) is a crowd-sourced listing of dataset related portals around the globe. In the GIS category, there are over 120 sites listed - mostly governmental (e.g., federal, state, county, municipal, etc).
This question is one my organization (a large Midwestern research university) is actively engaged in. We've been focusing our attention on OpenGeoportal (OGP), but as we're in many ways a very Esri-centric place, a lot of our development effort has been implementing support for working across Esri products to the open source side (previewing ArcGIS Server services in OGP, for example). I'll focus my answer on OGP and our experience with it.
I think a big question is how much developer time you have access to. We've been able to implement OGP with basically one part time graduate research assistant doing the coding and sys admin bits. Having Esri support would be attractive in some ways, but I so far I think having the code base open for hacking and adaptation outweighs it.
OGP utilizes a Solr index on the Java (Spring framework) backend, which is very speedy with tens of thousands of records, and via sharing potentially scales really well.
With Solr on the backend, the real power of OGP, and the aspect seeing a lot of active development (this, for example), is the ability to harvest metadata from other instances of OGP. With a robust governance structure currently in the works, there's an eye towards a future of a federated system of OGP instances sharing metadata across institutions and really improving the discoverability of spatial data.
That said, there are always concerns about FOSS projects running out of developer juice and dying on the vine. Yet, its embrace of open standards would make any future move out of OGP far easier than could be the case with more "secretive" solutions on the list.
A small note is that the state of Minnesota is launching a CKAN powered geospatial commons this summer, hopefully tying together what had long been a laundry list of different agencies housing and distributing GIS data in myriad ways. You can view a BETA version here.
