If you are interested to become an Open Design Alliance associate member you better hurry. This membership is free this year but will cost you $250 next year.
The DWGdirect library supports the formats DWG, DXF and DWF.
The ADTdirect library provide support for Architectural Desktop custom objects. A beta version of Civil 3D libraries is also available
The DGNdirect library supports the MicroStation file formats.
You may incorporate the Open Design Alliance libraries into programs that are used within your organization or school, or are distributed for free to the general public via the Worldwide Web. You may not, however, distribute these programs in any commercial fashion, or in conjunction with any other commercial software applications. (This membership level is also appropriate if you are developing commercial application programs incorporating our libraries, but are not shipping. At such time as you are ready to ship, you may upgrade to a higher membership level.)
ODA is still don't offering a managed code solution for .NET but are planning to to add that in 2008.