In SQL*Plus it would be '.', but this doesn't work in PL/SQL Developer.
Marco, could you please add this functionality?
Here is description from SQL*Plus guide:
SQL*Plus treats PL/SQL subprograms in the same manner as SQL commands, except that a semicolon ( or a blank line does not terminate and execute a block. Terminate PL/SQL subprograms by entering a period (.) by itself on a new line. You can also terminate and execute a PL/SQL subprogram by entering a slash (/) by itself on a new line.