Ikari Warriors (Multiplatform)

Cross-Series (Athena): Athena is an item that grants bonus points if you can find her. (Credit: Mike Waters)
Status: Unverified

Illusion of Gaia (SNES)

Character Cameo: On the Diamond Coast you can find Turbo, a dog that Enix has used in a lot of their games, such as Soul Blazer and RoboTrek. (Credit: Shadow Villgust)
Status: TRUE: Verified by FO.

Cross-Series (Soul Blazer): The first boss of Soul Blazer, "Solid Arm", is the exact same boss of the Gem's Mansion in Illusion of Gaia. The boss itself says that Will was stronger than Blazer at the end of the battle. Blazer was the name of the main character of Soul Blazer. (Credit: Bomberguy221 & Loto)
Status: TRUE: Verified by FO. Note: Some people have emailed me to say that these games are sequels, and therefore, this shouldn't count as a cameo. Indeed, I've often seen Illusion of Gaia touted as a sequel to Soul Blazer, but this is really the only plot thread that connects them, and this happens entirely in a bonus, optional portion of the game. Therefore, I believe that Gaia is about as much a sequel to Blazer as Blazer was to ActRaiser. Therefore, please do not send me any more corrections to this entry. CASE CLOSED!

Programmers/Cross-Series: The Phoenix is the ultimate power in this game, just like many other Enix games. (See under ActRaiser 2 and Soul Blazer for more info.)
Status: TRUE: Verified by FO.

Literature (Hamlet): Hamlet the pig is named after the Shakespeare character. There is even a line in the game regarding Hamlet that is a spoof of "To be or not to be".
Status: TRUE: Verified by FO.

Literature (Hamlet): Another Hamlet reference: If you look at the enemy chart that came with the game, you'll notice that the yellow skulls of Mt. Kress are called "Yorrick"! This game's writers must like Shakespeare. (Credit: ACC Kain)
Status: TRUE: Verified by FO.

Commercials/Products (Rolex): The Rolek company is probably a take on Rolex. (Credit: ShadOtterdan)
Status: Coincidence?: I doubt it's coincidence, though I also suspect the Rolex company wouldn't be too happy about this "connection", considering how Rolek in the game made its fortune...

Cross-Series (Soul Blazer): The snake ball enemies from Soul Blazer make an appearance in the Sky Garden. (Credit: ShadOtterdan)
Status: TRUE: Verified by FO.

Immortal, The (NES)

Programmers: If you make your way through the Red Room of Worm Doom without using the sensor, you can climb down the ladder at the end and find a room with the programmers of the game in it. They'll give you a coffee pot that will automatically wipe out two bosses later in the game. (Credit: Kairobi King)
Status: TRUE: Verified by FO. Source: Nintendo Power.

In the Hunt (Multiplatform)

Products (Toyota/Nissan): The billboard in level 2 has a picture of a car with the word "TOYOSAN" above it, a combination of Toyota and Nissan.
Status: TRUE: (Screenshot) proves it.

Iron Tank (NES)

Cross-Series (Ikari Warriors): The man climbing into the tank at the start of the game is Paul from Ikari Warriors. (Credit: Andrew Rae)
Status: TRUE: (Screenshot) proves it.