Minor spoilers ahead...

The link between game has been confirmed in quite a bit of detail by Valve. Portal 1 takes place some time between HL1 and HL2, and might have been caused in some part by the 7 hour war. Aperture Science was a rival company to Black Mesa that originally sold shower curtains to the military. The billions of dollars made from those sales helped Aperture Science to kick off the "Take-A-Wish Foundation", the "Heimlich Counter-maneuver", and the "Portal Project".

The only direct link in-game that we have between the two games is the Borealis (mentioned a lot in HL2:EP2) which was an Aperture Science research ship that "disappeared" along with part of the dock. The events of HL2 seem to point that it had been recently found (perhaps through a slow-teleport, much like what happened to Freeman and Alyx) in an arctic location, and it will obviously be the focus of the next HL game. The posted Portal 2 achievements seem to make reference to a boat, which is likely the Borealis, but we don't know what time period that the game is taking place. (obviously after the borealis incident, but unknown if it will be before HL2, or after)

It's been said that the events of Portal 2 take place hundreds (200?) of years after the events of Portal 1.

http://www.halflife2.net/forums/forum.php

A good site to check out for theories, mythology, etc. if you're into the HL and Portal franchises.

However, when the player is next woken, an autonomous P.A. system reports that the time elapsed is "99999...99...". It is unknown what unit of time it refers to. The presence of unrotted paper, wood & upholstered furniture throughout the game's environments would seem to support a timespan of decades (ex: 9999 days, or 27 years) rather than centuries.

If it is 27 years, that would put Portal 2 a few years after HL2. Maybe it will be more clear in HL3, or with the DLC coming soon.

hl2 was boring?.......

am disappoint sir

HL2 was amazingly fucking boring. I tried to play it recently just to see the ending and I couldn't do it. Honestly how fucking long can you walk around linear areas with nothing happening before you get bored?

well, i played it before i got into quake :D

There's a boring part in the middle, but once you get back into city 17, it's awesome again. The episodes are really awesome too cause they are quick and full of action, no boring parts.

last part is quite cool, middle parts can drag on.