A very cheap small chip, compatible with Arduino.
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Pro Many variants available
Many different variants are available depending how good you are at soldering or how small it needs to be.
Pro Very small
Can fit inside small devices.
Pro Very cheap
Without case can be below $5.
Pro Has ultra-low power mode
Pro Has bluetooth and wifi modules
Con Some variants require soldering
Not all variants include easily plugable pins. Usually only the more expensive variants include these.