Well, I'm not sure if they have been recommended yet, but I suggest <a href='http://www.fanfiction.net/s/2760303/1/The_Best_Seven_Years' target='_blank' rel='nofollow'>The Best Seven Years.</a> Plot Summary: After over 10 years since moving to Britain, Calvin has to explain to Suzie just what he's been up to. Crossover with Harry Potter.
My thoughts: This is extremely well-written, and puts Calvin and Hobbes in character EXACTLY, and lovingly depicts the crazy antics they would get into while in Hogwarts. Another good point is that our favorite duo rarely if at all get involved with the antics of the main cast. They have their own story, and it's HILARIOUS. Also, despite having a lot of humor, the author isn't afraid to bring forward a serious plot.
And it's Prequel, <a href='http://www.youtube.com/watch?v=PmeM4N2eHhY' target='_blank' rel='nofollow'>In Love and War</a>. Plot summary: In Calvin's attempt to do a report on the United States Civil War. In the process, they accidentally go to the Beast Wars era, where they meet the Maximals and Predacons and get stranded.
My thoughts: The story has just gotten started, and the updates are few and far in between. Still, like the first it is well written, and the hilarity of Calvin and Suzie's misadventures through time before the main plot kicks in is a must-read. Also, Calvin and Suzie both have Hidden Depths, which I look forward to more.
Oh, and the second, despite being a prequel, has next to nothing to do with the first. Comes very highly recommended.