We developed the Universal LDAP Connector (ULC) to communicate with our LDAP servers. We built it from the ground-up, conforming to RFC 2251, entirely in VB.NET code.
We needed such an object because we communicate with AIX, ActiveDirectory, and Novell LDAP servers from our Web applications and the ActiveDirectory objects that come with .NET didn't make the cut (specifically, they couldn't write to the server).
We submit this library in the hopes that the on-line community will be willing to extend it. It currently meets all of our needs and, so, we are not motivated to build it further.
This library is free and open. We do not offer support, but are certainly interested if you find any undocumented bugs.