These may not be quite as popular in fiction at the moment, but if you change the fantastical word "portal" to the more scientific "wormhole", the successful tv shows "Lost" and "Fringe" seem to be exploring that.
I completely agree with the "His Dark Materials" mention above. That would be my favourite example of the portal genre. I can't think of any that haven't already been mentioned for my best/worst; my thing as a kid was time-travel stories, which aren't too far off, I suppose. The fish out of water aspect was certainly one of the appealing factors, and also the ability to learn about another society "first-hand" as it were: the protagonist and I were both experiencing the world for the first time.
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I completely agree with the "His Dark Materials" mention above. That would be my favourite example of the portal genre. I can't think of any that haven't already been mentioned for my best/worst; my thing as a kid was time-travel stories, which aren't too far off, I suppose. The fish out of water aspect was certainly one of the appealing factors, and also the ability to learn about another society "first-hand" as it were: the protagonist and I were both experiencing the world for the first time.