The DS1820 was made obsolete and replaced by the DS18s20 which is almost the same and usually works as a direct replacement.
There is another variant, the DS18b20 which can be cheaper and is physically the same. BUT it presents the temperature data slightly differently so may need a modification to the software of some designs if they do not state that they support it.
There are similar rtc chips to the DS1307 but not an exact equivalent as far as I know. Using a slightly different rtc chip would again need a change to the software.