Addns.pl is a self-contained DynDNS update client for DynDNS.org (and possibly others). It supports a variety of IP detection methods including external webcheck/router pages. It also supports multiple configurations and hostnames, smart abuse prevention, and custom/static/dynamic DNS, and can run as a daemon, commandline options only, or with a configuration file.
Heirloom mailx (formerly known as "nail") is derived from Berkeley Mail and provides the functionality of the System V and POSIX mailx commands. Additional features include support for MIME, IMAP (including caching and disconnected use), POP3, SMTP, S/MIME, international character sets, maildir folders, message threading, powerful search methods, scoring, and a Bayesian junk mail filter. Mailx can be used as a mail batch language in nearly the same way as it is used interactively. It can thus act as a mailbox filter, can fetch mail from remote accounts, and can send files as attachments.
The Internet Software Consortium DHCP distribution includes a DHCP client, a DHCP server, and a DHCP relay agent. DHCP is a protocol for automatically configuring nodes on an IP network. The ISC DHCP server and client are well known for being extremely configurable, reasonably easy to use, and quite efficient.
Metabase is a DBMS-independent PHP API to access SQL databases. It supports MySQL, PostgreSQL, Oracle, Microsoft SQL server, Microsoft Access, ODBC, Interbase, Informix, MiniSQL, and SQLite. It supports nested transactions, blobs, and prepared queries. It can install database schemas defined in a DBMS independent XML format that describe tables, indexes, primary keys, and auto-increment sequences. It is also able to upgrade a database to a new schema without affecting the data that was stored since it was installed or upgraded for the last time. There is also schema reverse engineering support.