Fan-made Art

Art can be found in many forms. People made comics and cosplayed, cakes and other content. This is a picture of a girl that I met at Anime Boston in 2008 who cosplayed as Chell, she even has her own portal gun.

Motivational poster

Chell

Turret

Weighted Companion Cube

Comics

VG Cats seems to have a special connection to Portal, they seem to like the turrets and GLaDOS's song.
These are other comics that are also portal related:
Mario
3 Portal
Is that cake?
Portal-fun

There are many more fan-made objects out there.  These are just a sample.