Service providers with SAML version 2 for SSO accessible to public?

UPDATE: Samling is live again at https://fujifish.github.io/samling/samling.html

Samling is a serverless SAML IdP for the purpose if testing any SAML SP endpoint. It supports AuthnRequest and LogoutRequest.

It runs solely in the browser to simulate SAML responses returned from a SAML IdP - no registration, no servers, just a browser. You can control many aspects of the response - from success to various failures.

Simply setup the target URL for the SAML IdP to be https://fujifish.github.io/samling/samling.html, and you're done. If there is a SAMLRequest query param present on the request, Samling will parse, extract and populate the relevant fields.

If you don't want to use the online version, you can clone the samling repo from https://github.com/fujifish/samling and host it yourself - all you will need is a static file server.

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.