Steemit is the first web interfaced platform built on top of the Steem blockchain.
As to the alternates, there are many others which is very similar to Steemit.com like:
Busy.org - https://busy.org
eSteem - https://esteem.ws
SteemPeak - https://steempeak.com
There are also others that are like decentralized copies of popular social sites like:
dLive (Twitch of Steem) - https://dlive.io
dMania (9gag of Steem) - https://dmania.lol
dSound (SoundCloud of Steem) - https://dsound.audio
dTube (Youtube of Steem) - https://d.tube
SteepShot (Instagram of Steem) - https://steepshot.io
Zappl (Twitter of Steem) - https://zappl.com