When LINQ shipped, it included a provider model, to encourage people to make other systems and data stores accessible from the coolness-that-is-LINQ.
Back in February, Charlie Calvert published a long list of LINQ providers. In July, Robert Shelton later reblogged it. Charlie also has a list of LINQ Links. Nice to see. By my count there are 33 different LINQ providers listed. I'll bet there are more by now.
You can also search for more on LINQ providers.