As of month 7, 2008:

LINQ to Amazon

LINQ to Active Directory

LINQ to Bindable Sources (SyncLINQ)

LINQ over C# project

LINQ to CRM

LINQ To Geo—Language Integrated Query for Geospatial Data

LINQ to Excel

LINQ to Expressions (MetaLinq)

LINQ Extender (Toolkit for building LINQ Providers)

LINQ to Flickr

LINQ to Google

LINQ to Indexes (LINQ and i40)

LINQ to IQueryable (Matt Warren on Providers)

LINQ to JSON

LINQ to NHibernate

LINQ to JavaScript

LINQ to LDAP

LINQ to LLBLGen Pro

LINQ to Lucene

LINQ to Metaweb(freebase)

LINQ to MySQL, Oracle and PostgreSql (DbLinq)

LINQ to NCover

LINQ to Opf3

LINQ to Parallel (PLINQ)

LINQ to RDF Files

LINQ to Sharepoint

LINQ to SimpleDB

LINQ to Streams

LINQ to WebQueries

LINQ to WMI

LINQ to XtraGrid

[http://rshelton.com/archive/ 2008/07/11/list-of-linq-providers.aspx]