Texpipeline allows the conversion of (La)TeX documents with simple filter pipes such as "cat input.tex | tex2pdf > output.pdf". It removes the hassle of running (la)tex again and again, leaving lots of auxiliary files in the current directory. This is especially useful for programs which create LaTeX files automatically and just want to present the PDF output to the user.
urlwatch is a script intended to help you watch URLs and get notified (via email) of any changes. The change notification will include the URL that has changed and a unified diff of what has changed. The script works out of a single directory, so there is no need to install anything. State files are kept in the same folder. The script supports stripping parts of a page that are always changing through the use of a filter hook function. It is typically run as a cronjob.
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).