For lovers of Six Degrees of Separation (or Six Degrees of Kevin Bacon): the obscure characters in jdorama.
Yasuda in Water Boys appears in Gokusen.I'm sure there are tonnes more...I just haven't seen that many dramas yet. ^^; Briefly, Densha Otoko is alright...way too painful to watch sometimes (k_chan, is this really what some otaku are like?), and I don't really like Itou Masaki, but I was watching it for another reason altogether... Gokusen wasn't bad, but a bit repetitive after a while...there's only so many times you can watch Nakama Yukie win in such a cliched manner. Kimi wa Pet is, to me at least, more enjoyable than the other two - but that's probably because I absolutely love the manga, which, other than the 'pet' thing, addresses some really pertinent subject matter wrt women and equality, especially in Japan (and maybe in other Asian countries?). Momo was really close to the manga character (I'll admit - MatsuJun did well in achieving that), but Sumire and Hasumi-san were changed perhaps a bit too much. Finally, Summer Snow isn't exactly the most original drama, and suffers from the 'exactly how many tribulations can one family go through' syndrome, not to mention the acting being somewhat forced half the time, but, well...Oguri Shun did really well in his role there.
The other female teacher in Gokusen 2 is in Kimi wa Pet.
Noda (Gokusen) is in the NANA movie...and I've already posted the screenshot...woops - it was meant to go with this entry...
Densha's sister is Nobuta, and Hermes's brother is in Gokusen 2 as well.
And a number of the Gokusen kids appear in Stand Up!! (which I..er...haven't actually seen...)
Also, I've finished Gung...I did say once that Yul was looking better than Shin...but I'll take that back. ^^;