If you don't like System.DirectoryServices, or if you want  a pure LDAP API, you can get third-party LDAP libs for C#.  Novell just posted an update to theirs last week, you can find it at http://forge.novell.com/modules/xfmod/project/?ldapcsharp
 
They say it is for C#, but of course it will work for any .NET language.
Source code is available, under the MIT license.   They also ship a binary DLL, but it appears to be built against mono.