The damage calculator is always a handy thing to have open. You can use it to see if you should switch out or stay in and do damage.
The Flying Press articles are always helpful and you can learn a thing or two there.
Smog Archives put several key Pokemon in the spotlight, helping you with teambuilding
Viability rankings are one of the most important pieces of info a competitive player can have, since it shows you which Pokémon are most viable in the current meta.
Metagame threads and Metagame discussion threads are also handy. You can get the latest buzz in the competitive scene.
If you have a Pokemon you want to use, always be sure to read its Strategy Dex entry, to see if it is viable or not.
You also have Pikalytics
Serebii.net's Pokemon of the Week have useful detailed analyses and sample sets to help you.