ThinkGeek offers the Aperture Science Portal 2 Interactive Gel
in Blue, Orange, or White for $1
for shipping. That's $2 under last week's mention and the lowest total price we could find. This gel can bounce, stretch, shatter, and more.
Of note, you can purchase a 3-Pack for $1.99 plus $6.04 for shipping. That's $7 under our previous mention and a 33-cent per-pack savings over buying them individually.