The captcha.php library is very user-friendly and extremely easy to integrate into existing forms, because of its simple API. It nevertheless sports many configuration options, like multiple image modes and accessible alternative text riddles. It also has "AJAX" super cow powers and many sensible defaults to frustrate users less than other implementations. It prevents spambot access well, even with its relaxed defaults.
pop3ml is a program to manage mailing lists and newsletters without a local SMTP server. It requires only a simple POP3 email account. It's highly configurable by users. You can subscribe or unsubscribe simply with email messages. There is optional subscription and unsubscription confirmation, subscription moderation, message moderation, digest message mode, parent/children list management, mail scheduling to postpone or repeat a message, a mail filter (with a procmail-like syntax), a message queue for unsent messages, message size check, a command to retrieve messages, bounce detection, message header management, black/white/allow/deny lists, and other functions. There is also a program to test list settings without sending any messages and another program to view and search through stored messages. You can configure it with programs like phpMyAdmin.
File upload and validation class is a PHP class that can be used to validate and process uploaded files. It can validate a given uploaded file by checking its MIME type or its original file type, and by checking its file size against a given limit and file type. It currently supports audio, video, PDF, and image file uploads. The class can create a path in the server as well.
TbsZip is a PHP class that enables you to read and write Zip archives. The class doesn't require PHP extensions or temporary files. TbsZip can read, add, delete, and modify files stored in an archive. It can even start with an empty archive. The original archive is not modified. The new archive can be saved in the server as a new file, or provided directly as an HTTP download, or even as a binary string. If the Zlib extension is activated in PHP, then TbsZip can automatically compress and uncompress files in the archive.