Seetxt is a lightweight text file and man page viewer for X windows. It maintains "document meta-data" for each user, allowing them to automatically keep bookmarks and highlights for read-only system files. It also has a "server mode" so that command line requests can be sent to a single running server rather than starting multiple instances. Other features include hypertext-style apropos (man page) searches, file monitoring, multi-level regular expression searching, a command history, and nice little toggle lights on the interface for the server and file monitor. There's also cross-application drag-n-drop support, and handy little features that allow you to view the output of shell commands and process the text buffer with an external command (sed, awk, grep, perl, etc.) Seetxt is POSIX compliant.
clac (Command Line Advanced Calculator) evaluates mathematical expressions input via command line arguments or from stdin and writes the results to stdout. Unlike other command line calculators, clac has infix (natural order) expression syntax, is quite comfortable with complex numbers, defines a great many functions and constants by default, and allows easy definition of new user functions and constants using Python.
Musca is a simple window manager for X allowing both tiling and stacking modes. It is similar to ratpoison but more friendly to the mouse and using a simpler keyboard navigation. There are no built in status bars, panels, or window decorations save for thin window borders to indicate focus. Window navigation can be mouse click to focus or entirely keyboard driven. Window tiling is manual but simple, and there are no restrictions on how you divide up the screen. It uses dwm's dmenu utility for launching apps and running various built in commands not mapped to hot keys.
Peggy helps you to create GTK applications that run out-of-the-egg. It offers functions to load resources (like .glade files, images, and locales) from a zipped or unzipped egg. It is a thin layer on top of setuptools and PyGtk. It doesn't do anything very fancy, but it can save you from reading some documentation.
Gaze is a lightweight Gmail checker that sits in the system tray and notifies you of new messages in your inbox. You can configure how often the program will use the ROME API to check the Atom RSS feed for your inbox, as well as optional combined notifications, custom browser actions, theming, and more.