Salesforce has a free developer edition you can sign up for at: http://developer.force.com. It will enable you to test with them acting as either a SAML 2.0 SP or IDP. It is quite simple to sign up and use its SAML features for testing purposes.
For Google, they offer free 30 day Google Apps accounts for trial purposes - beyond that you need to pay.
As you say - there are loads others (like PingFederate or OpenAM) that you could either get for free, sign up for a trial for or purchase - if you want something in house.
