There aren't many PDAs on the market anymore since they've been largely replaced by smartphones. And smartphones do involve monthly charges, unless you buy a GSM smartphone with no carrier plan and just use WiFi-- but that would be an expensive purchase without a contract subsidy.
For standalone PDAs with WiFi, the HP iPAQ is pretty decent and can sync to Outlook. You could also go with an iPod Touch. The iPod Touch is basically an iPhone minus the phone.
Here's our iPAQ review: http://www.mobiletechreview.com/HP-iPAQ-110.htm .