My favorite calendar application for Treo 650 is DateBk5. There are other options such as Iambic's Agendus, KSDatebook, A5, and others. Plus the built-in PalmOS applications of course. But DateBk5 works best for me, and not only does the developer give fantastic support, but gives away the profits to a gorilla sanctuary project. I tried to do without it for a while, and even tried to stick with just his WeekView product, but finally I just couldn't stand it anymore and went back to my favorite DateBk5, and it feels like "coming home!"
There was one surprise waiting for me, though. The newer Palm's have a field called location that I've become accustomed to using. It's very convenient. But when I started using DateBk5, I noticed all the locations I entered were not to be found. A little research turned up a note from the author indicating that noting is done yet with that field. But here's the tip for those of you that are synching with Outlook.
Sync your Palm device. Go to the calendar view in Outlook, and choose View... Current View... Active Appointments. You'll see all your (future) appointments with a column for the location. Manually cut and paste that location into the the appointment text. No big deal! I couldn't find it, but you could probably do the same with the note as the new location for the location text.
And this might be a bit out-of-date, but I saw a note from the author also talking about how location would be integrated into DateBk5 with the next major release. It quite excited me because it said roughly around the turn of the year, which is actually not that far off. I'm very much looking forward to more improvements in this already great program!