The primary reason I started using PDAs, specifically the US Robotics Pilot 1000, back in 1997 was to manage my calendar, contacts, and tasks in a mobile device rather than in the large Franklin Planner I carried around everywhere. This core PIM functionality is still important on mobile devices today and if a device doesn't perform these functions well then it usually doesn't succeed in the market. I actually manage my calendar and contacts from smartphones and then sync that data back to my hosted Exchange service and then down to my Macbook or Samsung Q1 so it is very important that I can use the PIM suites to their fullest capabilities.
A couple of years ago I discovered Agenda Fusion from DeveloperOne and Pocket Informant from Web Information Solutions. Both of these applications run on Windows Mobile Pocket PC devices and allow you to have a single application that integrates your calendar, contacts, tasks, and notes. This integration also includes the ability to connect the information so you can link contacts to calendar events and more. Both applications are extremely powerful and I personally haven't even been able to keep up with and use all the features they both have available to users.
There are some differences between the two applications (i.e. dual alarms and reminders in Agenda Fusion 8), but the only way to really know which one is for you is to download the trial and give them both a shot. The user interface is different and it really comes down to a personal preference since each is quite amazing. The good thing about having two super PIM apps like this available is that they keep one-upping each other with new product updates and releases. Pocket Informant 2005 was initial released on March 31st, 2005, with the latest update out in April 2006. Agenda Fusion 8 was recently released in April 2006 with the previous version released about a year before that. These two companies keep the heat on each other and I personally keep bouncing back and forth between the software on my Dell Axim X51v.
I just read on Pocket PC Thoughts that a new public beta of Pocket Informant 2007 was just made available for download and testing. I'll have to try this latest release out now too and see how it compares to Agenda Fusion 8. On my Palm devices I also was a big fan of iambic's Agendus application. There is also the DateBk5 software that competes with Agendus. S60 users also have a couple of choices for integrated PIM applications with Papyrus and AquaCalendar. There are other choices available for these platforms, but these are the major software choices I have found available.
Do you use an integrated PIM suite on your mobile device and if so, which one do you recommend to others?