I've paid close attention to Esri's promotion of their "GIS for everyone" resources at ArcGIS.com. Just go there, Esri says, and create your own maps by "mashing up" satellite imagery, street or topographic basemaps, census data, environmental data. And add your own lines, polygons, markers, and labels.
That works if you can don't mind using everyone else's data but your own.
So far, if you want to create maps on ArcGIS.com with your own data, you have to own ArcGIS Server. Or borrow one. Then publish your data as map services on ArcGIS Server.
You can't upload your own data to ArcGIS.com from Excel, or as Esri shapefiles, or as Esri layer packages, and add them to your "Map for Everyone" at ArcGIS.com. You can post those data files, and others can download them to display in desktop map viewers. But that's not what we're talking about here.
Thanks to Dr. Jack Ma at Univ. of Maryland, I still have access to their ArcGIS Server after graduating from the MPS GIS program. (Jack gets a little payback - and occasional class presentation from me.) Here's the REST directory:
Two ArcGIS Servers are better than one. Especially when Server #1 has limited tech support.
While working the past six months for the City of Philadelphia GIS Director, I also connected with Maurie Kelly, director of the PASDA geodata portal at Penn State. I'm grateful to Maurie for providing map service publishing support for Choptank River Heritage geospatial data. (Maurie's generous logic is: Choptank flows into Chesapeake ... Susquehanna flows thru PA into Chesapeake ... So, yeah, let's host it on PASDA.) Here's the REST directory:
Once you have map services published on ArcGIS Server, wow, there's so much you can do: ArcGIS.com mashups, AGXO map presentations, Flex map viewer from my own web site, data downloads, geoportal, geoprocessing services. Even output KML for Google-based services.