URL of their beta platform: beta.singularitynet.io
There are free services available on the Ropsten Testnet (Testversion of the marketplace) as well as the marketplace on the main Ethereum network – just switch between networks with e.g. Metamask.
More info about the SingularityNET platform:
Developer portal: https://dev.singularitynet.io
This SDK enables you to interact with the services registered on SingularityNet: https://github.com/singnet/snet-sdk-python
This CLI enables you to add your own services to SingularityNET: https://github.com/singnet/snet-cli
In chapter two of the Whitepaper 2.0 there is an explanation to how everything is tied together: https://public.singularitynet.io/whitepaper.pdf