Lots of great responses in this thread:
https://forum.inaturalist.org/t/current-or-general-favorite-field-guides/656
There was a website on fly identification that I often found myself using to help with family-level identifications but it doesn’t seem to be up anymore, which is a real shame. However, another one from my bookmarks that I might need to start checking more is from two iNat users (@edanko and @zdanko):
https://sites.google.com/view/flyguide/home
Living in South Korea, the resources I use focus on species present here. For example, the Birds Korea Gallery and 소금쟁이의 비밀 (Water Strider Secrets) for quick visual guides. Then there are the online listings from guidebooks when I don’t want to go to the library - or my bookshelf - to access an image, like 한국산 잠자리 목록 (Odonata List of Korea), 한국 밤 곤충 도감 (Night Insects of Korea) and 거미 생태 도감 (Field Guide to Korean Spiders).
I’ve also saved a few links/images from research papers showing species within a group to help with making identifications. An example is this comparison from ‘Taxonomic Review of Bombylius and Bomylella in Korea’ for beeflies.
