SI7021
SI7021 with no library
is I2C temperature and humidity sensor from Silicon Labs. I use their other products, like SI7051, which is very good temparature sensor.
You can get SI7021 from Ebay for very little money. The 7021 is typ ±0.3°C (±0.4 max) from –10 °C to +85 °C. And a bit worse beyond that. Not bad at all!
As for humidity does, it is ±2% RH (±3% RH (max)), 0–80% RH, and additional, up 1.5%, when RH goes from 80% to 100%.
I2C scanner sketch shows sensor at 0x40 (One thing I don't like is that all this sensors have the same and only one address of 0x40)
As far as sketch goes, I found one that has no library.
SI7021 visits: