Thanks for the replies, all.
To add to my own thread, I've always liked the concept behind the TV series Ghost Whisperer. As you'd expect from the title, life revolves around a woman who can 'see dead people' and help them find peace by helping them complete a final task. Not every ghost is friendly, of course, and some aren't even exactly clear on what it is they want. I would love to run a game like this.
As an added bonus, have one of the PCs actually BE a ghost. This works amazingly well in a d20 Modern setting and, for the right player, dealing with the benefits and complications of being constantly insubstantial is a campaign in and of itself.
Nice!
You could even work in a bit of Dresden Files.