Eve Fleet Simulator - Simulate fleet PvP and compare/analyse fits and hulls.GESI - Google Sheets script to interact with ESI.EVE Ref - EVE Online reference site with full item database and stats.
You can participate with CCP devs and third party devs by joining Tweetfleet on Slack and hopping into the #esi channel. Since Eve apps and websites come and go we will try to check each of the them periodically and keep the list up to date. This is an attempt to keep an updated and maintained list of some of the many great tools and applications.
There are those who play Eve Online using the game's given UI and there are those who manipulate the data and create tools to work within the Eve Online Universe.