Tools/Resources

PC Finder - A very simple tool that finds PC solutions. It groups solutions based on what piece is saved.

Sfinder - A very powerful tool that finds PC solutions. It has a lot of features that most people won't get use out of. I wouldn't recommend this unless you need more than just seeing if a PC is possible.

Box Solutions - Every possible solution for 4×3, 3×4, 4×4, 5×4, 6×4, 2×6, and 4×6 boxes

Setups

Best Chance Field - A list of every 4 piece setup for all 7 PCs. Some of these aren't practical due to how hard it is to spot solutions.

Perfect Map - A tool to find setups for all 7 PCs. It only show setups with a 95% or higher chance to PC.

Practice

PC Mode - A game mode on Jstris that will end if you don't get a 2 or 4 line PC.

Perfect Clear Challenge - A website to practice all PCO solutions.

PC Quiz - A website that shows queues for PCO and Grace System then asks whether or not it is possible to PC. It shows you the solution afterwards.

Other

four.lol - A very good website for Tetris guides. It has very good guides for PCs.

Nilgiri's Replays - Nilgiri is a very good player. His PC Mode replays are very good for learning setups.