- MyWi for tethering/mobile hotspot
- BiteSMS lets you send/respond to/view SMS messages without quitting your current app (press up on the volume control and then tap the volume icon to compose).
- Notifier, puts notification icons in the title menu and on your lock screen (notifications for email, missed call, voicemail, text messages, instant messages).
- SBSettings gives me quick one swipe access to turn off the phone, toggle 3G and other settings.
- Backgrounder + ProSwitcher for Palm Pre style multitasking
- 3G Unrestrictor for using Wifi only apps over 3G
- Netatalk/SSH access for custom theming
- MxTube lets me download and save youtube videos.
- Add vibrating haptic feedback to the keyboard
There are other reasons, but those are the ones off the top of my head.