"ArcGIS.com is a very good start. It really looks and feels modern. Runs snappy and is quite intuitive. My problems with it are few, but to me they are just killers. No support for any open standards. As long as you use ArcGIS Server Services and Layer Packages, you can share. If you don't have ArcGIS then this just isn't going to be the place for you. Even if you use ArcGIS Server and only share WMS services, you can't partake in ESRI's ArcGIS.com."