Chaos;Head and Steins;Gate exist in the same ("Science Adventure") universe, but on different world lines.
- Chaos;Head happens in Shibuya and the events happen one year before Steins;Gate
- Steins;Gate happens in Akihabara
- Robotic;Notes happens in Tanegashima Island and the events happen nine years after Steins;Gate
The events of Chaos;Head are consistent with the events of Steins;Gate within the Beta worldline, divergence ratio: 1.130426%. Steins;Gate and Robotics;Note exist on the "Steins;Gate" worldline, divergence ratio: 1.048596%. (Steins;Gate was also originally in the same worldline as Chaos;Head, which changed after Okabe sent the first D-Mail and passed to another worldline.)
There is a mention of "Knight Hart" which is the online persona of Chaos;Head's protagonist Nishijou Takumi, in episode 4 of Steins;Gate. In the Steins;Gate game, it's mentioned that this character was also looking for the IBN 5100 (the non-conical 8-bit version of the game gets more in depth into his motvations).
Kurisu explained the "Time Leap Machine" concept to Okabe using NOZOMI papers from Chaos;Head.
On big TV outdoors, FES's performance can be seen in the Alpha timeline. Giving the right reply to Faris's mail earns the player Ayase's portrait and FES's music, sung by Kanako Itou, which include the Chaos;Head OP, "Find the Blue."
There's a Seira-tan cosplay cameo in Steins;Gate as well:
Robotics;Note connects the two series a bit more by revealing that the "Committee of 300" are the driving force between the major events of all three series.
Nae from Steins;Gate appears as an adult in Robotics;Note. Daru and Kurisu show up on the Twipo network in Robotics;Note as "DaSH". In the VN game, Kurisu uses a tripcode. In the anime, Kurisu posts in @channel under the handle "KURI_KAME" in episode 15 (about 2D and 3D limitations).
Daru writes the program that eradicates Kimijima Kou from Iru-o.
SERN (and their projects) is also mentioned several times in both Steins;Gate and Robotics;Note