While I strongly recommend keeping local backups of your files on your computer, external hard drives, flash drives, CD/DVD-RWs, etc, it's also a good idea to back up your files online! Here's a list of file storing sites where you can back stuff up for free. Spreading your files across different storage sites may make things more complicated but can also keep from ever having to pay for expanded storage space on any of them. Also, storing your files online creates an easy to share them with friends or publicly on the internet!
https://app.mediafire.com/
File storage site I've been using since the 2000s! They give you up to 50 GB of free file storage space.
https://www.google.com/drive/
Google Drive allows you to use 15 GB of online storage for free. You can also save links to other people's google drive folders in your main directory, making them easier to find again later.
https://www.dropbox.com/home
Drop box gives you 2 GB of free storage. Much smaller than some of the others, but every bit counts when you want to save money!
https://www.4s.io/
4shared gives you 15 GB of free storage space!
https://mega.io/
MEGA will give you 20 GB of storage space free!
https://archive.org/
Internet Archive is a very good place for storing things such as writings, art, audio, software, videos., etc. All posts are public so this site is ideal for making backups of finished works that you want to show off/keep safe. They run off of donations so if you use their site for storage please consider throwing some cash their way!
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The Following Sites are places where you can archive your video game mods and maps!
https://www.moddb.com/
Mod.db is a nice community where you can archive and share your maps and mods, storage space is unlimited!
https://gamebanana.com/
Gamebanana is rather messy in my opinion, but is still another great place for archiving and sharing game mods/maps you have created!
https://steamcommunity.com/workshop/
Steam Workshop mods can't just be freely downloaded from a web browser, so I consider it inferior to gamebanana and mod.db. Still, it can be another good place to archive mods and maps you made for games that have workshop support! It does tend to ease the installation process for mods, but people who got the game from another store may not be able to get your mods!
https://www.planetminecraft.com/
Lastly, here is my favorite Minecraft File website - running strong since the early days of Minecraft. If you've ever made a skin, map, mod, etc for Minecraft, you can probably archive and share it here!