This site on Github has lots of interesting projects that will help you improve your C programming ability.
Whether it’s a chess engine, a sudoku solver, space invaders clone, tic-tac-toe (noughts and crosses for us Brits!), Othello, program a text adventure plus a lot of non games stuff as well.
I’ve added this to the C code Links page.
Or do you fancy learning how to write your own virtual Machine in C? Authors Justin Meiners and Ryan Pendleton have crafted a 14 part tutorial that will teach you how to write a VM that can run assembly language programs.