Here's a list of all the posts in the building an IQueryable provider series.  If you want to get the latest and greatest source code just follow that last link.  If you want to discover how to build your own IQueryable provider read from the beginning.  It is sort of like a tutorial.

Part I - Reusable IQueryable base classes
Part II - Where and reusable Expression tree visitor
Part II - Local variable references
Part IV - Select
Part V - Improved Column binding
Part VI - Nested queries
Part VII - Join and SelectMany
Part VIII - OrderBy
Part IX - Removing redundant subqueries
Part X – GroupBy and Aggregates
Part XI - More of everything
Part XII - Relationships and the IQ Toolkit
Part XIII - Updates and Batch Processing
Part XIV - Mapping and Providers
Part XV - Transactions, Sessions and Factories
Part XVI - Performance and Caching
Part XVII - Automatic Caching and Loading Policies

The source code found in this series is also available at CodePlex.
http://www.codeplex.com/IQToolkit