Notepad: That lightweight Notepad what come with N900, but please add Save As to menu.

Calculator: That what come with N900.

The one of the reasons why I change from N950 to N900:
H-E-N (USB host mode GUI): http://talk.maemo.org/showthread.php?t=65232

fMMS: https://wiki.maemo.org/FMMS

Backup: Have to be compatible with that Backup what come with N900

Maps: Or any other navigation software

Evopedia: http://evopedia.info/

Desktop Command Execution Widget: https://wiki.maemo.org/Desktop_Comma...Widget_scripts

Calendar Widget: That what came with N900

Location Widget: That what came with N900

And these when you have time:
Midnight Commander, pCam, RAW viewer, MyPaint, gFTP, Filebox, Gnumeric, AbiWord, Fennec, Hatari, fMSX, mBarcode, Milkytracker, Pierogi, Tuner, HDR Capture, PDF-reader, Radio.