Borealis
The Borealis is a lost mysterious ship originally owned by Aperture Science before they lost funding to Black Mesa. It's stranded in the ice in the North with its drydock and a mysterious weapon.
The Borealis was originally to be part of the 
Half-Life 2 storyline, but it was scrapped.
Portal in Half-Life Continuity
This is a picture seen at the end of Half-Life 2: Episode Two. There is a container labeled Aperture Science, so there definitely is a link between the Half-Life universe and Portal.
