Useful Tools

Web Tools

Arcath has some useful tools including a building planner, creep designer, console viewer, and memory viewer. Check it out at screeps.arcath.net

The League of Automated Nations is a site to track alliances and their rankings

Libraries

There are also many useful tools the community has written and placed on GitHub.

Screepers Organization has a ton of tools, libraries, and other goodies

ScreepsMod Organization contains a large collection of mods for Private Servers

Others

There are many other tools, sites, and libraries out there for screeps, more specific lists exist on the official docs site