There are quite a few alternatives and I've actually written free ebook on the subject entitled "Online GIS - Meet the Cloud Publication Platforms that Will Revolutionize our Industry". You can download it for free at: www.onlinegis.com
Here's a tweet length summary of the products covered in the book:
ArcGIS Online: Comprehensive feature list. Jack of all trades, master of none. Confusing pricing. Card carrying Esri users will love it.
CartoDB: A GIS programmers wet dream. The power of PostGIS and Mapnik but none of the setup headache, all wrapped in attractive packaging.
GISCloud: Online alternative to traditional client/server GIS setup. Many features but hampered by a frustrating user interface.
GeoCommons: The place to share your data and use the data of others. No coding required. Slick UI, great visualisation tools.
MangoMap: The quickest way for GIS users to publish web maps. No coding required. Lots of map features, slick UI that’s geared towards simplicity.
MapBox: Making maps sexy again. Programmer focused. Great for maps that need to fit a brand and be able to scale for high traffic.