I was reading [Richard Downer's excellent response](https://retrocomputing.stackexchange.com/a/2158/2155) about how Amiga libraries work and one sentence in particular got my attention:

> 
> Yes, a pointer to exec.library is always at $4, and this is (in
>  theory, but not in practice) the only fixed address you should ever
>  use in AmigaOS.
> 
> 
> 

I only ever knew about $4! So what are other addresses/data in a fixed memory location in AmigaOS?

(Of course, I'm not talking about hardware registers; this question is specifically about software related addresses and memory areas)