People have been trying to solve Valve's "Second Test" ever since the G-Man picture was found in the preload files. Today it was(seemingly) solved by Halflife2.net users.

At the Half-Life2.com Media Page, it was discovered that the 6th picture changes to the G-Man's face upon mouseover. When you click it it takes you Here

After you insert 'nova' as a username, and 'prospekt' as a password you are taken to a Black Page with a text box dominating the middle of the screen. The source of the page has an ascii art of Alyx, with reference to whatsmyname.jpg

If you enter the name of that creature(Barnacle) into the text box on the Secret Page, you get to see new art work for Half-Life 2.

Since there is no 'Congratulations Halflife2.net', some users speculate that the test isn't over. Only time will tell

You can read more about the ongoing efforts Here