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.
Pandoc is a Haskell library for converting from one markup format to another, and a command-line tool that uses this library. It can read markdown and (subsets of) reStructuredText, HTML, and LaTeX, and it can write markdown, reStructuredText, HTML, LaTeX, DocBook, OpenDocument XML, RTF, ODT, GNU Texinfo, MediaWiki markup, and S5 HTML slide shows. Pandoc extends standard markdown syntax with footnotes, embedded LaTeX, and more. A compatibility mode is provided for those who need a drop-in replacement for Markdown.pl. Included wrapper scripts make it easy to convert markdown to PDFs and Web pages to markdown documents. It has a modular design where the addition of a new input or output format requires only the addition of a reader or writer module.
Asynchronous DNS Resolver for Haskell is a library that provides an asynchronous DNS resolver on top of GNU ADNS. Not all options are supported, but A, MX, and PTR lookups work nicely. There is also support for retrieving generic RR types, CNAMEs, and for NSEC zone walking. The library can be expected to work with fine ADNS 1.4 or later. It might also work with version ADNS 1.3, but that hasn’t been tested.
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.
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.
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.