How to list Extended Properties for all databases
You could run the command in a cursor over the databases:

DECLARE @DbName nvarchar(500), @SQL nvarchar(1000);
DECLARE C CURSOR LOCAL FAST_FORWARD FOR 
SELECT QUOTENAME(name) from sys.databases order by database_id;
OPEN C;
FETCH NEXT FROM C INTO @DbName;

WHILE @@FETCH_STATUS = 0
BEGIN
    SET @SQL =
    N'
    USE '+@DbName+'
    SELECT '''+@DbName+''' as databasename ,name, value 
    FROM fn_listextendedproperty(default, default, default, default, default, default, default)'
    EXEC SP_EXECUTESQL @SQL;

    FETCH NEXT FROM C INTO @DbName;
END
CLOSE C;
DEALLOCATE C;