4chan
9gagBlockchan - https://www.blockchan.com/ [not yet available]
Agar.iodMania - https://dmania.lol/
BitlySteemgar - https://steemgar.herokuapp.com/
ImgurSteemly - https://steem.ly/Steeming - http://www.steemimg.com/
Instagramd.pics - https://d.pics/
LinkedInSteepshot - https://steepshot.io/Busy - https://busy.org/ [same as SteemLinked]
MediumSteemLinked - https://steemlinked.herokuapp.com/ [same as Busy]
PacManSteemit - https://steemit.com/
QuoraSteemPacman - https://pacman.steemliberator.com/
SoundCloudCrowdini - https://crowdini.com/
TwitterDSound - https://dsound.audio/
VenmoZappl - https://zappl.com/
YouTubeSteempay - https://steempay.org/DTube - https://d.tube/
Steemir - https://steemir.com/ [automatically post YouTube videos to steem]
Other notable
https://steemnova.intinte.org/ - browser based game
https://steemprojects.com/projects/p/steempress/ - x-post WordPress to steem
https://www.peerhub.com/ - online marketplace
https://spelmakare.se/steem/rps - rock paper scisors
https://socialwallet.io/ - integrating cryptocurrency into social networks
https://steemconnect.com/ - connect to services w/o sharing private key
Posted from my blog with SteemPress : https://blog.coryagroup.com/2018/06/23/steem-powered-alternatives-to-popular-services/