Some programs I find useful that are in reasonable shape for general consumption. Those that don't sound inherently portable probably just work under Linux.

si - Interactively invoke system calls and library functions.

ike - Send alerts to your mobile phone when your IMAP account receives new mail.

mint - Minimal X toolbar.

refun.c - Replace libc functions with your own on the fly.

mm.c - Rudimentary ncurses mp3 player front end.

include.el - Emacs command to insert the right #includes for library function calls.

rfc.el - Emacs command to retrieve and cache RFCs.

dir-ftp.el - Emacs command to visit FTP sites by content.

bless - Temporarily give a process root privileges. Includes a point-and-click bless for X clients, but I haven't gotten around to posting all the clunky trappings it uses.

sock2proc - Given a description of a TCP connection, find a process that has one of the sockets.

hijack.c - Force a process to load a library.

nock - Simple sockets client and server for testing and measurement. (Some horrible documentation)