Single Player - Hidden Goals
Final Transmission - One of the most hidden ones in my opinion. In Test Chamber 6, there will be a radio that comes past as part of the cloud of garbage that GLaDOS throws at you. Save the radio and then carry it up with you into the Rat Man's hidden den. It's close to the exit, behind some panels. If you bring the radio in with you, then you'll hear his transmission and get the achievement.
Preservation of Mass - You get this one by saving the companion cube in Test Chamber 7. Just use a portal to bring it up with you after you open the exit.
Good Listener - Another decent joke one. As you escape from GLaDOS with Wheatley, GLaDOS will ask that you please go into the final chamber. If you use a portal to leave the ramp, or get to the final room in any other way, she'll seal you in and gas you, which nets the achievement.
No Hard Feelings - During the escape with Wheatley, you'll find some turrets on the "turret redemption line." There's one that's begging you to save it, noted that "I'm different." You can pick this turret up and take it off of the line to get a few more lines and an achievement.
Scanned Alone - When you remove the template turret during your sabotage efforts with Wheatley, you can just walk into the scanner for an achievement.
Ship Overboard - When you pick up GLaDOS in her potato form, you should be able to find a door hidden behind some random objects. Pull them aside to find the dock for the Borealis.
Door Prize -You'll find several large steel doors along the way marked as "vitrified." if you examine the door or the intercom, you'll get more Cave Johnson lines. I'm going to guess that most people miss the three in the Borealis docking area though. Make sure that you check the dry dock during your run to get them all.
Portrait of a Lady - For a look at the path, just keep your eye open for the locked off room as you leave the first test of the orange gel. If you go inside, you can find a portrait of Cave Johnson and Caroline.
You Made Your Point - For your first test under Wheatley, just refuse to press teh button and solve it. It seems to take a little less than a minute for the achievement to register.
Smash TV - You can smash the monitors Wheatley uses to communicate with you. If you crash into them, hold a turret in front of them or point a laser at them, they will be destroyed. You need 11 in total.
Pit Boss -To get the pit boss achievement, you need to wait for the beginning of the final chapter, "The Part Where He Kills You." When Wheatley calls for you to please come back to the pit that you started on, just be cooperative and run back. Wheatley will be surprised, and then go ahead and crush you.