I recently came across Romance Neolatino (Neolatin). It's an auxiliary conlang based on romance languages which aimed to be like a natural evolution of Vulgar Latin---that's what Wikipedia says, at least. I think that's a really neat idea.
I knew of Interlingua already and how understandable it was for romance language speakers, such as myself, but Neolatino is even more so.
These languages really aren't very useful, of course. After all, if two people want to understand each other, especially if they already speak languages in the same family, then they might as well just learn each other's language instead of diving into some conlang that nobody actually speaks. The benefits of learning Neolatino would be to kind of surprise people by speaking something that they don't recognize and yet understand? Maybe? Which is rather silly, of course.
I should really learn French...