Answer:
PlusKey.net is definitely the biggest site I have found so far for Reading Plus answers. It works like a forum so you can both submit answers and see everyone else’s answers, theres also the option to download a collection of all the answers right to your computer.
The main downside with this site is it can be confusing to get into. Sometimes they close registration randomly, other times it can be days before getting approved. When I signed up I had to submit answers into a form and didn’t get a reply back for 6 days. Eventually once I did get in and it was amazingly useful, theres even answers as old as 4 years back when reading plus was new.
They also have a ‘lounge’ area where they share Reading Plus memes and other funny stuff.
While its a bit tough to get in, once you get accepted it makes reading plus a breeze.