There are several forums similar to Reddit where users can post their own content and interact with each other. Here are a few popular ones:
Stack Exchange
Voat
HubPages
Snapzu
Aether
Hacker News
MetaFilter
Lobsters
Fark
Reddit (since it fits the description)
4chan
Something Awful
Newgrounds
Digg
Bored Panda
DEV Community
Product Hunt
Wattpad
Instructables
ArtStation
Blogger
Medium
Imgur Community
Goodreads
But remember, some of these platforms may have different rules, guidelines, and community cultures, so be sure to familiarize yourself with them before participating.