Achivements in the Orange Box on the Xbox 360.

I want to start a thread about achievements on the Xbox 360 for the Orange Box games.  Please post which game this achievement is from, and tips about it.  If its a hidden object, like the vortigant cave, tells us were it is.  If its get X amount of kills on Y enemy, were is the best place to do it. 

Please don't post for ones like get 1000 kills in TF2 or beat half-life 2.  All other achievements can and should be posted about.  And who do you get Terminal Velocity on Portal?

Half-Life 2

Half-Life 2: Episode 1

Half-Life 2; Episode 2

Portal

TF2

I think this should be in the XBox section, Terminal Velocity all you do is look straight up; shoot a portal; look straight down; shoot a portal; leave for 6 minutes; then you get it.

This is a complete achievement guide

Thanks Chupa

No problemo, always happy to help! ^_^