An open source, cross-platform e-book organization application. Also handles format conversion, conversion of RSS feeds into e-books, reader synchronization. You can even read your e-books with it.
A single-file database administration utility written in PHP. Supports MySQL, PostgreSQL, SQLite, MS SQL, and Oracle database servers. Formerly phpMyAdmin. Seems to let you do everything you could do from a database shell. Session-aware. Has plugins for a number of popular web apps to help you fix their databases should you need to. Easily themeable, just drop a new CSS file into the directory Adminer is installed in.
A VNC client implemented entirely in HTML5, using WebSockets and Canvas. Supports encrypted connections. Remotely manage servers without having to install extra software!
RackTables is a web application that makes it easy to maintain inventories of your data centers. Can handle multiple data centers, racks, machines, categories and types of device... requires a basic LAMP stack.
How to display the list of file shares on a Windows server.
How to interactively edit the mail queue of a Postfix server to delete messages, clean up junk, or other stuff. Requires administrative access. Read the whole thing before you do anything.
howto use ansible to configure and maintain a workstation.
A utility that uninstalls all of the cruft from Windows that turns it into a 15 gigabyte disk hog right off the DVD.
A utility which monitors routers by logging into them, pulling the config files, and comparing them against known-good files to see if anyone's been monkeying with them. Supports Cisco, Juniper, Foundry, Redback, ADC, MRT, Alteon, and HP gear, probably more.