OSI – The official website. You will need the client from here. Most of their wiki will also be relevant as InsaneUO mirrors current day OSI for the most part.
UOGuide – A fan-run wiki for OSI shards. Mostly accurate and good for general knowledge but some things can differ due to out of date pages or custom items/changes on InsaneUO.
UO-Cah – Primarily an in-depth guide to animal taming, training and all things related, this site also has in-depth guides to most dynamic events (Treasures of, Jolly Roger etc). These events sometimes work differently on InsaneUO but the rewards are often the same. Pet details should match unless otherwise specified in the Taming section of this Tome of Knowledge.
Stratics Wiki and Stratics Forums – The oldest fansite for UO. The wiki can be a good source of information for anything that mirrors OSI. The forums are fantastic for up to date news and a lot of discussion around builds, items and play.
InsaneUO Discord – Template-Ideas channel – Guides written by our very own community for getting started, gathering gold, character builds and training your pet.