Since I split my time pretty evenly between Windows (at work) and OSX at home, I'm forced to choose platform-independent solutions for my GTD tools.

Sandy handles all of my date- and time-based reminders. I've been using this product since it's earliest beta and I love it.

I use Jott occasionally for capture when I can't easily write things down on index cards (the Levenger Pocket Briefcase is one of my most prized tools). I call, leave a quick message and it's in my email inbox as soon as I get to a computer.

30boxes.com is my calendar of choice. It's quite simply the best web-based calendar I've ever used (yes, it's better than Google Calendar). The interface is snappy and it behaves exactly as you'd expect it to - very Mac-ish.

For projects, action lists (and other lists) I use a large squared moleskine.

Good luck in your search :)