The Stack Exchange network user interface has a lot of forms, and subsequently, inputs with various character limits in place. Some of them are notorious (such as custom moderator flag character limit or the custom suggested edit reject reason one), some are less well-known.
It is past time we had a definitive guide on those for the sake of having documentation and for providing a unified place for tracking changes to the limits (should such a thing occur in 6 to 8 time units), so the question is:
What are the various character limits across the Stack Exchange network?
The scope of this Q&A is limited to character limits only.
For a detailed guide on rate-limiting, see the The Complete Rate-Limiting Guide.