Yes; there is mine, for starters. ☺
I asked about X because mine is intended as a user-space replacement for the virtual terminal emulator that is built in to the operating system kernel. It's not an X program. Like the kernel's terminal emulator, it uses the framebuffer and the various human input devices.
So if the question is whether any terminal emulators exist, and X is not a consideration, then the answer is most definitely yes. My terminal emulator supports Unicode and 24-bit colour out of the box, and a slightly older version of George Nachman's
24-bit-color.sh script displays some pretty rainbows when run on it.
I'm running it right now, installed as binaries from a Debian package, on Debian 8. (There is no local X server on the machine.)
The Debian 8 packaged Konsole also appears to support 24-bit colour.