1. Used to introduce a sentence in which you express an idea you have, when you think that idea is very good, reasonable, appropriate or timely. English = 'I've got this great idea!', 'Hey! Why don't we...', 'Say, let's...' and so on.
2. Used to repeat someone else's suggestion favourably (ie, when you think it is a good idea too). English = 'That's a great idea....', 'What an idea!....'