I used to mess a bit with the ROM on my HTC Phone Edition devices and found that Shadowmite, a TreoCentral forum member, created the ability to customize the ROM on the Treo 650. I followed the great instructions and created a ROM that removed RealPlayer and Versamail and added in MyBible, PowerRun, Uninstall Manager, Documents to Go (just the main prc file), and Resco Backup.
I then installed Pocket Tunes, Agendus Pro, SplashBlog, MobiPocket Reader Pro, SplashPhoto, TealMover, VolumeCare, TideTool, and the DataViz Tech Tool (to move Docs to Go to my SD card). I then needed an email application and installed Agendus Mail 4.2, which I then found does not let me use Gmail due to the SSL limitation. I just purchased the upgrade to the SSL edition for $10.95 and am now set there. I like the way Agendus Mail lays out different email accounts and find the interface better than VersaMail. I was going to try to put Agendus into ROM, but it is updated too often to mess with that.
I also just saw a few pages with recommended apps and am going to install Profiles and KeyCaps600 soon. Do you have any other recommendations for software or ideas about what should be in the ROM?