The wiki is one of the most useful resources to be checking details like Riven dispositions and pros/cons to weapons.

This article is essential for any new players, and is a good reference in general. The table about damage is very useful for preparing adequately for missions and sorties.

This really useful fansite keeps track of a few different timers and shows current alerts etc. A good alternative to the mobile app, if you only need certain tracking features.

This is quite an important list for long-term players who have already acquired Relics that have been removed from the reward rotations.