Essential Server Addons List?

Well, to begin with, an absolute must is an Administration System (ULX/ULib are the best free combo, however you should take a look at a paid solution such as xAdmin2 & xLogs), as well as a reliable 3rd party anti-cheat (such as SwiftAC).

Then, well, adapt the addons to your needs - redesigned UI (chat, F4 menu, pop-ups and more)/printers/bitcoin mining/drug harvesting/scripted activities to earn money (mining, for example)/unique weapons & vehicles etc.