A handful of small, perhaps generally useful, software libraries in the public domain under the MIT license.
They can be found on GitHub at https://github.com/pem?tab=repositories .
A few examples:
hashtable: A simple string hash library in C. This was written many years ago but has been updated to modern C standards.
aatree: A C implementation of Arne Andersson‘s (now 30 years old) “Balanced Search Trees Made Simple”. This has better performance than for instance Red Black Trees, at the cost of a level counter in each node (instead of one bit).
string-metrics: A C implementation of string metrics like Levenshtein Distance (and variants) and Longest Common Substring. This is also available in a Ruby version.
For more, see the GitHub link above.