When adding donator ranks on a Minecraft server, there are more things to consider than you'd think. Not only do you need to meet the Minecraft EULA, but you also need to add perks that players actually want; or you'll end up getting very few donations.
So a few key points:
EULA: All donator perks must follow the Minecraft EULA, a summary linked here. A few highlights are that you can't restrict access to any part of a server, and you can't offer perks that change game mechanics for just donators (e.g. special kits just for donators, normal players must have a chance of getting them). If you use Buycraft, see this link for some useful information on how it might effect your store.
Quality over Quantity: Having a long list of donator perks can actually do more damage than good. When considering donating, your average player will only spend a few seconds glancing at the perks you offer - so make sure what they see is something they'd want. If you have just 2 or 3 perks that players really want, and they see these at a glance - every time they play on your server they'll have these potential perks in mind. If you have these alongside dozens of other less attractive perks users are much less likely to see them, and so less likely to donate. Perks that are visible in-game by other players can also be very effective such as trails or hats.
Nicknames: A very common donator perk several years ago that's still used on many servers is nicknames, the ability to change your in-game name whenever you like. 3 years ago this was great, as players were unable to change their names themselves. Now, this isn't the case. Players are now able to change their username every 30 days, and that change applies to all servers they play on. Not only does this make nicknames antiquated (outdated), but it also makes them very confusing. Imagine if 3 players not only change their account username but also change their nicknames over the course of a few days. These should be avoided and removed if you're currently using them.
Length of ranks: Something to take into account when adding donator ranks is how long players are going to be playing. If a player purchases a rank and plays on the server for several years, this can cause issues later on when you may have a large player base and few players donating - as they all already have ranks! An option is to have lower ranks last a certain amount of time, e.g. 2 months, and then have higher ranks last for the lifetime of the server - minimising the risk later on in the life of the server.
With these things in mind, here is our list of recommended donator perks:
/fly (In non-PvP game types, allowing players to fly can be a very attractive perk and still follow the EULA)
Trails (Options to customise the particle effect and duration can be used)
Hats (Allow blocks, banners and even mobs to be placed on player heads)
Pets (A mob that follows players around)
In-Game money (Ensure money can't be spent on perks that would significantly change mechanics like PvP)
/ptime & /pweather (Change time and weather for a particular player)
Extra friends, homes, warps, etc.
Use colour codes (Allow colour codes in chat and on signs, e.g. &1 for dark blue)
Talk without moving (Useful if many of your players use apps like MineChat, PandoraChatFilter can be used for this)
Server-wide reward (Allow donators to run commands that give perks/items to all online players)
Choose specific team (Restrict normal players to selecting a random team in mini-games)
Extra inventory space (Double the size of inventory for donators)
WorldEdit (On Creative servers, give donators access to a limited set of WorldEdit commands)
Disguise as mobs
We've only linked to specific plugins where features are difficult to find in general, just search ' bukkit/spigot plugin' in your favourite search engine to see available options for other perks.
An example of how these perks might be displayed attractively on a website can be seen here.
Simo389 has been running Minecraft servers since 2012, and developing for them since 2013. Currently he runs MineRifts, a YouTube channel and shares advice in these tips on how to publicise & improve your own Minecraft server.