Diary : Calender app (pre-installed) provides a very useful diary with day/week and month view as well as ability to set alarms for appointments.
Notepad/Todo List : Colornotes (free from market) is my personal choice, allows you to make post-it note type notes as well as tick-off to-do lists, can also be added as a widget to your home screen -
http://www.androidzoom.com/android_appl ... _bejc.htmlDatabase - there was a database system available on the market, but it seems to have since been removed, you might be able to find the apk using google if you do a little searching - the website for it is still up -
http://www.androiddatabase.info/Hope this helps!