The guy who created this flash game really did a superb job with the level design. Sentry guns (spies keep sappin' my sentry!) are strategically placed where if you don't use a companion cube or create a portal for them to disappear, you're going to get shot, and most likely die. The end-game levels are just ridiculous with moving platforms tat will lead you to death if you don't place your portals perfectly along the way.Aiming is real nice though, since you use the mouse over the screen and press either Q, E or left click for a portal. There are forty levels for you to mess around with and you are almost guaranteed not to beat this in one day. Don't worry though, this flash game will save your progress and provide you a great challenge.
Click here for a link to Portal via Newgrounds. I suggest playing the game in a pop-up version so your screen was freak out if you accidentally scroll up or down with your mouse. I'm currently stuck on level 38. No cake for me, yet.