AEditor is a programmer's editor for Windows, Unix, and Mac OS X that relies on the fox toolkit. It can do syntax coloring of Ruby and C++ files. It's easy to change the color theme. Most things are customizable. It is written in Ruby and is carefully unit-tested (it has 421 tests). It is written in only ~5500 lines of code, so it should be easy to grasp and extend.
Alexandria is a GNOME application to help manage a book collection. It retrieves book information (including cover pictures) from several online libraries, allows you to search for a book (either by EAN/ISBN, title, authors, or keyword), can import and export data into ONIX, Tellico, and EAN/ISBN-list formats, generates Web pages from your libraries, allows marking your books as loaned, saves data using the YAML format, features an HIG-compliant user interface, shows books in different views that can be filtered or sorted, and handles book rating and notes.
regexp-engine is a regexp engine written entirely in Ruby that makes it possible to search in text with advanced search patterns. It supports Perl 5 and some Perl 6 syntax and is fairly compatible with Ruby's native regexp engine, passing almost 98% of the Rubicon tests. The implementation is simple and contains no optimizations.
deplate converts wiki-like markup to LaTeX (standard classes, koma, dramatist, sweave), HTML/PHP (single page, chunked/website, HTML, or s5-based slideshow), DocBook (article, book, man/ref page), and really plain text. Currently supported input formats are viki and Ruby's rdoc. The viki markup supports footnotes, citations, index, table of contents, embedded LaTeX for mathematics, integration with R for dynamically generated figures and tables, and more. Output can be customized via page templates.