I am not sure whether there is any company that hosts silverlight apps for free, at least for the basic package usage.
Edit: I am aware that one just needs to include the Silverlight control in HTML, but first, not all hosting companies host .Net applications, and even less of them are hosting .Net 3.5, which Silverlights 3.0 is based. The percentage of companies that host this for free if you are in the range of minimum usage, is even less.
Any ideas?