じゃん is じゃないか, i.e., ではありませんか
When you say "すごいじゃん", you actually affirm "that's great, isn't it?", you give your opinion, and don't care about any one else.
The intonation is that of a strong affirmation.
だろう/でしょう is rather an introspective form, or an "open to suggestions" affirmation.
When you say "すごいでしょう", you say "that's great, isn't it" and you look at the people around you, expecting an agreement, a reaction.
The intonation is that of a question.
できるじゃん! is something you say while doing it: "see, I can do it!". Though more naturally, I'd say "できたじゃん" or "できているじゃん".
できるでしょう is something you say before he does it: "I'm pretty sure he can do it, can't he?"
(If I were to say "I'm pretty sure I can do it", I'd say it differently, that's why the subject changes in my example.)
Also, grammar dictates that you can't say すごいだろう, so usage has it like "すごいんだろう", when you want to leave politeness apart (even though でしょう as in the example above is perfectly ok in informal conversations).