I'm looking for a very simple wireless, low-power, low-cost switch that can transmit a signal to an Arduino. Ideally, each transmitter would have a signature. For instance, let's say I had 4 different transmitters that are attached to a sensor. When the sensor is tripped, I want to wirelessly send a signal to the Arduino. I've looked at these 433 MHz modules, but it seems like they would also need a microcontroller to send the data, while that's a possible solution, I'm wondering if there is a even smaller, simpler solution.
There is no such thing as a transmitter that can attach direct to a sensor, since no two sensors in the world work the same way. You need something intelligent between the sensor (to read the sensor value) and the transmitter (to tell it what to transmit).
There are "all in one" modules, like the ESP8266, which have an MCU and WiFi combined in one chip which are closer to what you want, but it is still down to you to program those modules to work how you need them to work for your sensors.