Iron Sudoku's non-Flash interface (yes, that's AJAX you smell) is easy to use and efficient enough for speed-players. It also has two different color markers, red and green (hold Shift or Ctrl, respectively) and lets you save your progress at any point (and I'm told autosave is coming soon). Most interestingly, Iron Sudoku is social software, so while you're playing you can chat with players around the world who are working on the same puzzle, and sometimes it's nice to know you're not the only one pulling your hair out hunting for hidden subsets. There's one puzzle per day and completing a puzzle wins you points—10 for Expert, 5 for Hard, and so on (but rankings unfortunately won't be enabled until April 1, unless I've been fooled). Signing up for Iron Sudoku is free, but paying members (a one-time $14.95 fee) get a few extra features like a friends list and account customization.