I use a PDA (Palm TX) with a Bluetooth GPS as a plotter using
Pathaway (also for windows mobile) .
Its not free (60US)but I have used it very successfuly for the last three and a half weeks working along the Eastfriesian Islands. Very useful when trying to avoid the 'no go' bird and seal reserves with no markings but a hefty fine for straying in.
You scan in paper charts and then calibrate using two or more known points. I have managed it easily so that the accuracy is very good indeed (a few meters - much more that that of the maps considering that the sands are always moving).
No costs for extra maps and I like it that whats on the plotter corresponds to my paper maps so I can use both easily together.
Difficult to read screen in sunlight but you can do it, but I leave it in cabin and there its fine.
I also have a little etrex for the DSC, the palm was to just be a bit of fun really, and I use the paper maps most, but it has turned out cheap and really useful.
The other advantage is I use the palm for loads of other stuff on board - mp3 sound system, reading ebooks, playing games, watching DVD's etc -(a tidy small solution on a small boat), as well as all the other stuff from work and home.
Another simpler and free one with basic COG , SOG etc is
Cetus
Hope this helps
PS sorry seems Cetus is only for Palm OS
PPS SBC sent me a link to Palm GPS info, it seems there is an equivalent
Pcket PC page on the site