lhs2TeX is a literate programming tool. It is implemented as a preprocessor that generates LaTeX code from literate Haskell sources. It allows for and provides different styles for the formatting of code. You can easily select between representing operators with mathematical symbols or with ASCII approximations, as well as deciding whether or not to highlight keywords. The formatting of your own defined tokens may be adjusted. Preprocessor-style conditionals are supported, and Haskell can be used to generate parts of the document.
The Glasgow Haskell Compiler is a robust, fully-featured, optimising compiler for the functional programming language Haskell. GHC compiles Haskell to either native code or C. It implements numerous experimental language extensions to Haskell for example concurrency, a foreign language interface, several type-system extensions, exceptions, and so on. GHC comes with a generational garbage collector, a space and time profiler, and a comprehensive set of libraries.
Postmaster is an Internet mail transport agent (MTA) that handles incoming ESMTP network connections and delivers accepted messages to the user's mailbox by piping it into an arbitrary local mailer (such as Procmail). It is configurable beyond anything you'll ever need, handles any number of concurrent connections without forking or using OS threading, and it is pretty efficient. It currently lacks any form of queue management, but for leaf sites that don't need to do extensive mail relaying, it is a reliable and powerful solution.
Darcs is an advanced revision control system. It has two particularly distinctive features which differ from other revision control systems: each copy of the source is a fully functional branch, and underlying it is a consistent and powerful theory of patches. In spite of its power, darcs is simple to use, in part because of the symmetry that is restored by making each copy of the repository a branch.
datapacker is a tool to group files by size such that they fill fixed-size containers (called "bins") using the minimum number of containers. This is useful, for instance, if you want to archive a number of files to CD or DVD, and want to organize them such that you use the minimum possible number of CDs or DVDs. In many cases, datapacker executes almost instantaneously. Of particular note, the hardlink action can be used to effectively copy data into bins without having to actually copy the data at all. datapacker is a tool in the traditional Unix style; it can be used in pipes and call other tools.
twidge is a client for microblogging sites such as Twitter and Identica (identi.ca). It is extremely versatile, can be customized to work the way you want to work, and can be combined with other tools to do just about anything. It can be used interactively from the shell. It is useful directly as-is, and with simple shell aliases can make a highly efficient system to do exactly what you want. It is perfectly capable of being your only client for microblogging. It can also be used in an automated way, via cron(1), or it can even integrate with your email system.
xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard; a mouse is optional. xmonad is extensible in Haskell, allowing for powerful customisation. Custom layout algorithms, key bindings, and other extensions may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several physical screens.