I want to share my custom map with TBNRfrags, but I'm not sure how to do it. Does it take a special plug-in, or site? It would be great if there where a world ID, or something, that I could just give to him. But if there is not, I'm open to anything else.
You go into %appdata%.minecraft\saves if in Windows, ~/Library/Application\ Support/.minecraft/saves in Mac, and ~/.minecraft/saves in Linux first and find your map.
Then you should compress it into a ZIP file, as it will not only be probably quite large, but most cloud drive services don't allow you to upload entire folders at a time.
Then you just need to upload your ZIP file to a cloud drive such as OneDrive, Dropbox, or MediaFire.
Finally you just need to get a referral link from your cloud service and email or otherwise send it to your friend
Hope I was helpful!
An easy way I use to do this sort of thing is MediaFire, which is a very easy-to-use site where you can store all sorts of data. It lets you upload any file from your computer (In your case you would choose to upload your minecraft map from %appdata%, .minecraft, then saves) and then create a link (shareable with anyone) where you can download that file.
As for the reliability of this site, I'de say it's good. At least, it has never failed me before if I have ever wanted to share files, and has always lived up to my expectations.