Unlike other solitare type games, this was more of a puzzle, as all the cards were exposed, and the game was usually winable (occasionally when I didn't think it was, the X-windows solver was good at producing a more authoratative ruling, though not always in my favor). Sometimes the solutions were especially tricky, but I was persistent and almost always would solve one in about twenty minutes.

I haven't played seahaven in a long time, but recently, I ran across a pointer to a new puzzle called SuDoku.

You start out with the red numbers only, and the rules that each row, each column and each 3x3 square can only use each number (1-9) once. From there, logic dictates where all the other numbers must go. Sounds easy, and some of the puzzles are pretty much that easy. But some of them aren't. You get to a point where the easy answers have all been found, and you have to start looking more far afield for the solution. There are plenty of sites that talk about strategy, so I won't get into it too far here, but some supposed "medium" puzzles really had me straining my brain to find the answer.

I had filled in quite a bit on this puzzle, but had reached a point where I had to fill in something in the lower right square, and was just racking my brain trying to figure out what the answer was. I even broke down and used the solver at the Daily SuDoku draw page to check my answers so far and to tell me what the next step was. It told me the next step was a 1 in the middle bottom of that square. But How? Finally I scored up everything I knew about that square from the others and this is what I saw:

So each of those other four squares had two possibilities, but more interesting, was that those four numbers had to go in those four spots. So that left the other square for the number one. I finally convinced myself of that logic, and the rest of the puzzle solved itself, but what a reach. We'll see if I can pull it off next time without the computer helping me. Today's puzzle wasn't much of a challenge.

[links]
the Daily SuDoku featuring a puzzle a day, archives going back several months, a solver, and good links to other sites.
BBC: The puzzling popularity of Su Doku
Wikipedia: a good introduction and many links & references

Update 2020:
The people at Longwood have closed up shop, so Seahaven for classic MacOS is not available anymore and even the product pages have been pulled, but I received a comment from Tomas with a link to an online playable version. Load up on your ad-blockers if you're easily distracted, but otherwise its quite fun.