What was L's real name?
According to both the Death Note Character list on Wikipedia and the L character page on the Death Note Wikia, his full name is L Lawliet.  He does have several aliases that sound more like real names but L seems to be his actual name.

From the L Wikipedia page:


  Tsugumi Ohba, the writer of the series [...] For L's name, he wanted to use a single letter with a lot of significance; he considered "I" and "J", but eventually chose "L" after careful consideration.


The Wiki page also points out that his name is written in Latin characters as "L"