There are at least two RTC modules containing the DS3231 available from China. Both can operate with the Raspberry Pi and one is designed specicially for it.

One useful feature of the DS3231 other than keeping the time is it's built-in temperature readout, however, initial tests suggest that this is inaccurate.

This device is as simple as it gets. There are two pull-up resistors for the I2C lines (4K3), a battery connected to the VBAT pin and a capacitor on the power rails.

This device plugs directly into the GPIO header.

This module contains the RTC and an EEPROM. It tends to ship with an incompatible CR2032 battery and as per advice on many web sites, it's desirable to disable the charge feature of the module (see schematic).