The power and capabilities of laptops are sometimes limited as described above. But in turn, they have a feature which desktops don't have, their mobility. I try to give a survey about applications which make sense in connection with laptops. Since I couldn't try all of them, there is currently little documentation. If you can provide further material, please contact me.
I'm not an expert in this field, so I just mention the tools I know. Please check also for other applications. Besides the usual tools tcpdump, netcat, there are two applications I know, which may be used to analyze network traffic:
The Multi Router Traffic Grapher (MRTG) is a tool to monitor the traffic load on network-links. MRTG generates HTML pages containing GIF images which provide a LIVE visual representation of this traffic. Check http://www.ee.ethz.ch/stats/mrtg/ for an example. MRTG is based on Perl and C and works under UNIX and Windows NT.
The CMU-SNMP package, is designed to configure and analyze the SNMP protocol http://www.gaertner.de/snmp/ .
Though designed to work from a single floppy, the Linux Router Project (LRP) , seems useful in combination with a laptop, too.
When thinking about the powers of laptops, hacking and cracking networks comes easy into mind. Though I don't want to handle this topic here, but instead recommend the Security-HOWTO.
If you are giving lectures, readings or presentations in different places, a laptop might suit your needs. You can combine it with an overhead display, a beamer or a second monitor. For a second monitor or a beamer make sure it is supported by your laptop. BTW though Microsoft's PowerPoint is often used for such things, there are also Linux solutions. See the software maps at KDE (K-Office)
http://www.kde.org and GNOME
http://www.gnome.org/ . Or the commercial packages Applixware
http://www.applix.com/appware/linux/slideshow/gfx011.html and Staroffice
http://vesta.astro.amu.edu.pl/Library/Linux/LinFocus/May1998/article15.html. MagicPoint or mgp, is an X11-based presentation tool. The home page is
http://www.Mew.org/mgp or
ftp://ftp.Mew.org/pub/MagicPoint/ or
http://jiji.mew.org/mgp/ .
A Linux laptop can be used to collect data outside an office, e.g. geodesy data, sales data, network checks, patient data in a hospital and others. There is support for wireless data connections via cellular phone modems and amateur radio. I am not sure whether PCMCIA radio cards are supported, see Aironet Wireless Communications http://www.aironet.com/. There are also laptops available with cases build for a rugged environment (even waterproof laptops).
With KDE http://www.kde.org (K-Office), GNOME, http://www.gnome.org/ and the commercial products WordPerfect, Staroffice and Applixware http://www.applix.com/ Linux has more and more business software applications. With the corresponding hardware, e.g. a portable printer and a cellular phone which connects to your laptop, you will have a very nice mobile office.
AFAIK there are currently three methods to connect a digital camera to a laptop: the infrared port (IrDA), serial port and maybe USB. There are also some auxiliary programs for conversion of pictures, etc.
AFAIK there are currently two methods to connect a video camera to a laptop: a ZV port and maybe USB, but I don't know how this works with Linux. I have heard rumors about using a sound card for video data transfer to a Linux box, see
http://worldvisions.ca/~apenwarr/ . I have heard rumors about a Linux-QuickCam-mini-HOWTO, but couldn't find a reliable URL yet. Check the sane package which is build for scanner support, this should contain support for still-grabbers as well.
If you have a ZV port in the laptop, it should be easy to connect it to a TV set, using either NSCA or PAL, but I don't know whether either works with Linux.
AFAIK there are two methods to connect a cellular phone to a laptop: via the infrared port (IrDA) or via the serial port. See the Linux/IrDA project for the current status of IrDA connections. AFAIK only the Ericsson SH888, the Nokia 8110 provide infrared support.
From the Hardware-HOWTO I know there is Trimble Mobile GPS available. You may also connect a GPS via a serial port. Most GPS receivers have a data port and can connect to a PC with a special serial cable.
AFAIK laptops are used in HAM contests. Please see HAM-HOWTO.
Together with an antenna and software like seesat or sattrack you can use a laptop to locate a satellite for visual observation. You could also use xephem on a laptop when stargazing.