10 Best sailing apps (android)
Android market https://play.google.com/store?hl=en

Many of the articles covering sailing apps are very US focused. As US charting, weather and tide information does not apply anywhere else in the world, we are keeping a close eye on apps that have broader applications. After all, cruisers and racers tend to sail all over the place whether on their own boat or OPBs (other people’s boats).

For Android (as of March 2012)
1.   Navionics Charts
2.   NOAA Buoy and Tide Data
3.   Passage Weather
4.   Tide Graph
5.   EarthNC (with Google Earth integration)
covers US, Bahamas, New Zealand and Brazil
6.   PredictWind
7.   PocketNav (integrates with ActiveCaptain)
8.   Marine Charts
9.   Google Sky Map
10.   Boater’s Pocket Reference

Here are some other neat apps:
Ship Finder
RYA Handy Racing Rules
Nautical chart symbols
SAS Survival Guide
Currents
Buoy Weather
Weather Eye
Marine Traffic
Memory-Map (NOAA Raster charts)
MxMariner
Marine Charts HD & Lite
NutiCharts Lite
AveTides
Tides Planner 10
Tide Graph
Tidal Chronoscope
History of sailing
Tide7 (UK, Ireland, Benelux)
Marine Traffic
Racing Rules of Sailing (US Sailing) [Costs €4.43 and has some pretty bad user ratings.]

Vice Commodore, OCC 

Graham and Avril Johnson put me onto this.

Download Google Earth and then link in a GPS feed either from your GPS enabled device or your NMEA network (using for example Franson GPSGate).

I have found the results outstanding, giving me confidence to sail places that I don 't have the chart coverage particularly in reef strewn areas.

In principle you need an Internet connection (GPRS/Edge is good enough) but not necessarily, Apparently, GE caches about 2GB of downloaded maps so if you do your planning research properly (at all relevant zoom levels) the data remains available offline. You do need to be sure you are off line otherwise GE will overwrite the cache when it can, leaving you with a hole in you chart locker when you need it!