Algorithms for Parallel Processing

Hey, I just wanted to know how many algorithms there are that cannot be accelerated by parallel processing. I know one such algorithm is Euclid's Algorithm (for GCF). does anyone know any other algorithms that cannot be accelerated by pp? if so please list the names and a general sentence of what they do.

Homework?

no, research project in computer security (see my other thread relating to my research, I can't post URLs yet)