"ID" or "Id" on User Interface
Solution 1:
According to Merriam-Webster, the abbreviation is "ID". If it were a correct abbreviation, it would have to be "Id." with the period.
Solution 2:
Personally, I use "Id". The compiler doesn't care but my eyes do. Compare:
GetIDByWhatever <-- looks terrible
GetIdByWhatever <-- oh so pretty!
Aesthetics is more important than grammar when it comes to code, always. (Update: 4 years later, I don't stand by this statement anymore)