I used to be a hardcore HTC man, but defected to the Dell Streak and find Android an awesome operating system. The Android Market has all the goodies. Below is a list some of apps I use.
Alarm Clock Plus - Set the alarm, and when it goes off you have to correctly answer a maths question to snooze or switch off. You can set the difficulty to the required level.
ASTRO File Manager - Think windows explorer for your phone
Blogger-droid - for mobile blogging
Compass - Does what it says on the tin
DCSwitch - lets you switch off the data connection, saving your battery life
Tiny Flashlight + LED - Lets you control the LED to you as a torch
Handcent SMS - improves the txting experience
JuiceDefender - battery saver - tweeks settings to save battery life
InkPad Notepad - (for notes) - simple app to take notes
Lookout Mobile Security - Antvirus for your phone
MyPhoneMyRules - turns phone silent and switchs off vibrate by placing phone face down, along with selecting individual contacts to keep on silent.
RockPlayer Lite - for playing video
Tango Voice & Video Calls - think facetime (apple). It works over mobile data connection also.
Unified Remote - lets you control you PC. Mouse and keyboard functions present.
WhatsApp Messenger - free SMS via wifi/data