Also the name of a children's show on PBS a few years ago which promoted tolerance and literacy.

A multi-racial group of kids (complete with ethnic names like Jamal and Alejandro) solve mysteries with the help of their friend "Ghostwriter."

Ghostwriter is, of course, a ghost, although no one knows his original identity. He can only be seen as a little glowing ball (similar to the one used on the Sing-a-Long Songs Disney collection), and only by members of the Ghostwriter Team. The catch, and what makes this a PBS show, is that while Ghostwriter can't see or hear anything, he can read and write, so communication with Ghostwriter was always through written messages. Often he would bring them clues in the form of words written in strange places, or word puzzles. I saw my first cryptogram on this show, when the team cracked a "secret code" to find a meeting by the bad guys.

While very heavy-handed, it was a pretty good show, actually interesting to me when I was a child. Unfortunately, the creators decided mysteries weren't enough, and filled the show with a bunch of sappy subplots about the characters. I stopped watching when two of the characters kissed.

Below are some additions from other members:
From Emoin: There's a line of books based on this series.
From Mindbender: Ghostwriter could only form words from letters already in the room. This was pretty cool, as you got to see the letters from one sentence rearrange themselves to make a completely new one. Also, I apparently didn't make clear how amazingly cheesy this show was. Rest assured, you'd have to look pretty hard for something sillier.
From blubelle: This series made a music video too, titled "Believe" or something similar.

My own memory for silly bits of nostalgia.