CustomDNS is a modular DNS server written in Java. It's based on Brian Wellington's dnsjava pacakge, but it adds support for virtual hostnames, SQL databases, and dynamic client updates. You can use CustomDNS out-of-the-box as a clone of ml.org/dyndns.org, or you can use it to develop custom DNS applications.
adlook (Address Lookup tool) is a multi-threaded host name and technical contact lookup tool. It accepts a list of IP addresses from the command line, from a data file, or from stdin, and outputs the hostname and technical contact for each address. Although it is written in Python, for the time being it runs only on UNIX systems. Python must have been compiled with threads enabled to run adlook.
PowerDNS Backend Development Kit allows a developer to code backends which can be loaded at runtime by PowerDNS, a nameserver that reads data from different backends. The stock distribution comes with MySQL and PostgreSQL and other backends. It is also possible to do failover, loadbalancing, and geodirection from a backend.
updyn is a shell script that updates a dynamic DNS service with the current IP address. It will work with any dynamic DNS service that allows you to call a URL to update the DNS. It works behind a router when an external IP address is different from internal IP, optional minimum time check, does not update unless your actual IP is different from your assigned IP, and can be set to update the DNS every set number of days whether or not your IP has changed.
Twisted Names is both a domain name server as well as a client resolver library. It comes with an "out of the box" nameserver that can read most BIND-syntax zone files as well as a simple Python-based configuration format. It can act as an authoritative server, perform zone transfers from a master to act as a secondary, act as a caching nameserver, or any combination of these. The client resolver library provides functions for querying all commonly used record types as well as a replacement for the blocking gethostbyname() function provided by the Python stdlib socket module.