Many problems require repetitive algorithms like network flows or suffix arrays. Whenever I need such an algorithm, I just copy-paste the same well-tested implementation.
This is my personal collection of algorithms and data structures, which have been in use during the last few years on hundreds of problems and two ACM-ICPC world finals. My favorites are FFT, suffix array, and convex hull.
I hope they will inspire you to write your own library or serve as a learning resource.
Enjoy! Let me know if you have any feedback or questions. :)