Reddit.com/r/pkmntcg
An online message board dedicated to the competitive aspect of the TCG.
Pokebeach.com
News on the card game, previews and discussions of upcoming cards.
Limitlesstcg.com
Tournament placings, deck builds, metagame analysis, deck builders, match simulators, online tournaments.
Trainerhill.com
Statistics and metagame information, deck comparison tool that lets you determine what cards you are missing.
Trainingcourt.com
Track your online and in-person match statistics and win rates with different decks.
Tcgmasters.net
Play against yourself, puzzles.
Pokecabook.com
Japanese tournament results and analytics. (Use a translator)
Sites.google.com/view/sv-block
Out of date card legality list, some useful information and tips.