At the very least Portal is certainly guaranteed to not overstay its welcome. I also like to think that Aperture Science is within the same continuity of Half Life. Even without the reference it seems rather plausible.
No Mutants Allowed
TheAdversary
Hassknecht
Portal and Half Life are in the same universe according to Valve. Portal takes place after Half Life 1 and before HL2, Portal 2 is very far in the future.