Photopea - The most advanced online Photoshop site. Takes a bit of time to figure out, but it can do everything Adobe Photoshop can, including editing animated gifs as well as saving edits to Imgur. 

Ez Gif - Online video to animated gif converter and editor. I recommend using Photopea to edit gifs as it's more advanced and has better quality, but this is a great site to use regardless. 

Photo Mania - Filter and editing site.

Pixlr - Filter and editing site.

Lunapic - The old trusty photo filter and editing site since Myspace. It's outdated today but people still like to use it.