I have a handful, most I played with and didn't use much.  I do use iSailor as a nav / gps.  The electronics on my boat are outdated, and so I prefer iSailor to my GPS / Nav.  It's really good and pretty reasonable.  The app is cheap (if i recall correctly), and you just buy the charts of the area in which you're navigating.  It's nice because you buy it once, and it works on all your devices.  It also loads the entire chart, so if you lose cell service it still functions.
I also use MarineTraffic - it's basically an AIS app.  I find it very handy and have used it many times crossing shipping lanes to see the course and speed of large vessels.  It's also nice to have their name in case you need to call.  It does require data to work, so it wouldn't be helpful far offshore, but it works for me in the 30nm crossing to Catalina Island from MDR.
The ASA app is cool... not much functionally, but is a compass and gps.  You can also send your current position and speed to someone via email.
If you're a BoatUS customer, their app is handy to have too.
I have Windfinder Pro - seems like it's pretty good, but haven't really used it.  Good for predicting / seeing different wind speeds and directions in your area.
Great topic - I'm looking forward to seeing what apps others like.

Cheers...
 

I am not a apple fan so I use a android product.  For apps I use Windfinder pro, Marine weather, Weatherbug, Accu weather,  active captain Pocket Grib. I have a older Garmin chart plotter and a older Ray marine radar. For tides and currents I get that info from my garmin. 

We use Predict Wind, Marine Weather by accuweather, and Skipper (charts, route tracking). We use the basic version of Predict Wind to determine good sailing days; and the wind  map to see the difference in wind speed and direction from various distances from the coast. Often we can find a good slot of wind for day sailing using the wind map.

Garmin Blue Chart, tide, wind, current, wave heights, USA vector charts and Bahamas Explorer charts, Active Captain.  Excellent app for $30

The Explorer charts. Are they just on C-map? I have the paper chart books. Also what app was $30

Garmin BlueChart Mobile, Apple App Store $30, iPad only.

Garmin BlueChart Mobile, Apple App Store $30, iPad only.

Windyty.com if you have internet is handy.  I use it on an android tablet.  Mainly it uses gribs,  but it makes allowance for physical features.   Any way, it looks cool!

FREE APP -- FREE CHARTS
 
We have a Garmin 4210 on Southern Comfort which works well, but I prefer a free software program, OpenCPN 4.0 for navigating.  When you are in a narrow, twisting, poorly marked channel, it is comforting to have one person drive as another studies the boat's position and assists.  OpenCPN has better resolution than the Garmin, is terrific for trip planning, and has better features.
 
I have a 7 year old ACER netbook (Windows), a USB GPS puck, and an inexpensive mouse which I use to run OpenCPN.  The charts are free from NOAA and updated regularly.
 
 
OpenCPN 4.0
http://opencpn.org/ocpn/download
 
NOAA
http://www.charts.noaa.gov/

I have an iPhone. The PO of our boat recommended Navionics. It is reasonably priced and does a nice job, based on my limited experience. I also have SailFlow and the BoatUS app. 

Here's my Boating apps for the iPad/iPhone.

DragQueen (recommended)
US East (has Active Captain data)
Navionics (integrates with my Raymarine E127)
Raymarine RayControl
iNavX
Windfinder
WindAlert
PredictWind
Bouys
BoatUS
Weather Underground (shows storm tracks)
The Weather Channel
Earthmate (goes with my Delorme satellite text device)

Take the wind prediction apps with a large grain of salt.  :)

iNavX doesn't stitch the NOAA charts together and I am fortunate enough to be sailing on a boundary.  Just makes it a hassel.  I pretty much just use the RayControl app and Navionics for my charting and trip and route planning.

I do have some of the Active Captain apps but they're, IMHO, not very useful at this stage.  All I care about is finding an anchorage, marina or getting some local knowledge.  I continue to use the old US East Navionics app as they appear to have removed Active Captain from the new app.  Sort of odd?

Since I'm pretty new, i don't know that many, but I do use Windfinder, Windyty, Environment Canada and  Navionics. It carries a lot of good info.