This is what I currently love using on my tablets:

Advanced Backlight - combines brightness and volume control in one applet and gives a very fine grained brightness control

Power Launcher - provides icons on the home screen for launching applications

maemo-pan - connects to the internet via cell phone with Bluetooth PAN (unfortunately no longer working out of the box on Diablo)

FB-Reader - THE eBook reader for the internet tablets

MediaBox - finger friendly snappy media player with support for a wide range of multimedia formats (through osso-media-server and mplayer) and, on the N800, FM radio


And if you're a developer or power user, there are two essentials:

becomeroot - enables "sudo gainroot" to become super user without the need to switch to R&D mode

openssh - log into the tablet from a computer, browse the filesystem via SFTP