This is a pretty good video on some of the developer options for customizing SharePoint Online. If you are not familiar with SharePoint Online, it is SharePoint hosted by Microsoft for use by small organizations or companies as well as large ones. As this video explains, there are two different flavors Standard and Dedicated. Development in the Standard offering is a bit more challenging since the current offering does not allow you to deploy code to the multi-tenant infrastructure. I liked the content of this demonstation, but the presenter could have been a bit more prepared.
You can try out SharePoint Online at http://www.microsoft.com/online
And the video I am talking about can be seen here: https://channel9.msdn.com/pdc2008/BB53/