Open Source reference implementation of LDAPv3. Check it out!
Netscape/iPlanet Directory Server
web2ldap even supports some extra features for these.
Novell eDirectory 8.x
With sub-tree renaming and some other neat features.
LDAP server in Lotus Notes R4.5x and 4.6x
Sometimes behaviour of this LDAP server was very weird making it
almost unusable. LDAPv2 if I remember correctly.
LDAP server in Lotus Notes R5.x (sometimes weird)
Most times it works quite well. Persistent connections are
dropped very fast - depends on the configuration I guess. web2ldap
contains a work-around for the null-byte terminated string
of the namingContexts attribute in Domino/LDAP's RootDSE.
MS Active Directory
web2ldap displays the GUID as active LDAP URL links.
Many Netmeeting directory servers (most times DNS alias
starting with ILS) contain invalid NUL-terminated strings in some
rootDSE attributes. This can't be worked around easily without
breaking LDAP implementation.
I also did not find a ILS server which allows non-exact searching
or any other useful stuff for general LDAP use.