There's a number of different OS X tools that let you sync text files onto your iPod and have them in a nice little group on your computer too. The only one I can think of off the top of my head is VoodooPad (a personal desktop wiki application, if you're not familar with it) and it even keeps the wiki links you've set up on the computer.

The only real downfall I could think of is that you have to wait til you're at your computer to update something. That's fine for capture, but it would start to bother me if, for instance, an appointment I had changed time. There's no way to make a note of that on the iPod. But maybe you can keep track of that stuff in your head. I sure can't.

If you don't need an application to keep track of your text files, just turn on disk mode for the iPod and drop text files in the "Notes" folder. There's a FAQ on the apple site somewhere about the formatting you can use in the files - they allow basic hyperlinks and such.

As for syncing calendar and contacts, on OS X calendars and to-do lists from iCal sync onto the iPod and contacts from Address Book. The settings for both are accessible from iTunes in the same set of preferences where you choose how to sync the music.