MissingH is a library of all sorts of utility functions for Haskell programmers. It is written in pure Haskell and thus should be extremely portable and easy to use. It also has no prerequisites save those that are commonly included with Haskell compilers. Highlights of MissingH include a full logging infrastructure, printf() implementations, various string and I/O utilities, a FTP client library, and more.
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.
hslogger is a logging framework for Haskell, roughly similar to Python's logging module. hpodder lets each log message have a priority and source be associated with it. The programmer can then define global handlers that route or filter messages based on the priority and source. hslogger also has a syslog handler built in.
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.
xtee (cross-tee/expanded tee) is a program for building complex pipelines. It resembles the tee command, except that instead of copying stdin to stdout, it copies an input file to stdout and stdin to an output file. You can use xtee for building things like a bidirectional HTTP filter (using netcat and sed).