A colloquial contraction of 'じゃないか' used sentence-finally to seek agreement or to assert something; casual speech.
それ、いいじゃん。
That's great, isn't it?
今日暑いじゃん。クーラーつけようよ。
It's hot today, isn't it? Let's turn on the AC.
もう終わったじゃん。後で話せばいいよ。
It's already finished, you know. We can talk later.
彼、来ないって言ってたじゃん。
You said he wouldn't come, didn't you?