“A line will take us hours maybe;
Yet if it does not seem a moment’s thought,
Our stitching and unstitching has been naught.” — W.B. Yeats

See this for general software issues. Some mathematical software may be found here. Here are some noes on computer languages.
Some programs in Scheme and Scheme modules, sort of.
This is promising.

Specific Code and algorithms:

Generalities

Organized by language