Hey, It's That Voice!: Um...everyone? Seriously the show has so many single episode focus characters and has been running so long, that literally every Japanese voice actor has been on it at some point.
You can also hear several familiar voices in the dub.
It is interesting to note that a significant number of voice actors that voiced recurring characters in Detective Conan had voiced important characters in Legend of Galactic Heroes before:
Of special mention is The Kidnapping of a Popular Artist Case (episodes 81-82), where the Victim of the Week is Minami Takayama, the lead vocalist of Two-MIX who also happened to be the voice actress of Conan. The Celebrity Paradox is immediately lampshaded when the cast noted how similar their voices are.