PowerQuest is the only  point'n'click engine that is:

• Developer oriented
• Based on a modern, widely adopted technology
• Used by its creator do publish real games
• Open to modification
• Wonderfully supported by its creator, who's friendly and always keen to help
• Free (but it's well worth paying some beers to the Powerhoof's guys!)

It's still young compared to some older (some way older!) alternatives and it may have rough edges, but the support is top notch and the flexibility is simply endless.

If you are a developer, I strongly advise supporting this project and adding Unity + PQ to your toolbelt.