What kind of backcountry trips do you want apps for?
For me it's more about being out quite a number of days (or more), so the mix of apps I focus on changes. Please take the below thoughts in that context, as apps that I'm interested for backpacking trips include some standard in-city type apps.
For example, on the CDT this year I found that Facebook was the way that current-year hikers best stayed in contact with each other, so I've got their android app installed. Having some newspapers that I like to read was nice in town. I don't know what's best there; I've got my local city paper plus BBC News and USA Today. The former is a pretty cool newspaper app. The latter I wouldn't normally read, but the app is pretty easy to use.
A good camera app is ideal. Turns out that the camera app that came by default with my Droid X was buggier than I had expected. I've started using Camera ZOOM FX, but haven't used it enough to know how much I'll like it yet.
Good app(s) or sites for weather forecasts is a good thing.
In terms of battery life, I like an app called Battery Indicator.
I suggest that you get Switchpro or something like it. In any event, I strongly suggest that you have a place at the far lower left or right of your screen that you can touch when you've inadvertently left set your screen brightness to 'low' the night before and now you can't see the screen at all in bright sunlight. Knowing by heart just where to push to make the screen brighter can be quite helpful.
Switchpro is worth the money, though, just to have a very easy and configurable way to control which services are eating battery life. So for example, I can turn on airplane mode without turning off wi-fi, or of course turn off wi-fi separately too --- everything is right there, including bluetooth, GPS, and screen orientation.
A good wi-fi finding app can be helpful in trail towns where you have poor or no cell service (or perhaps even where you have great service depending on your cell service plan details). I don't know what's "best", but I've got Wifi Analyzer.
I like having a voice recorder app handy. I've had good luck with VirtualRecorder, never a problem with it after lots and lots of use, but others might be as good or better, dunno.
I ultimately installed the Kindle app when I found the eBook version of a guidebook via Amazon.com that I wanted to take along on a trip, otherwise I use Aldiko.
I do use Gaia GPS, but have limited experience with it. I too have found it a little frustrating to work with, but that might just be me not yet having spent the time to really get to know it (?).
I have no idea if or how well these really work to limit battery drain, but FWIW I have installed both JuiceDefender and NoBars. In the latter case, I tend to explicitly turn my cell phone reception off anyway, so this might just be of more help to me "in town".
Consider something like Skype if you anticipate being somewhere that wi-fi is available but cell service is not (it happens). Or if you have overseas friends, or if your particular cell plan has limited minutes, or ...
At least for Verizon customers, take care not to use the Verizon-installed version of Skype, I would (and did) download the one from the google android market and use that.
I use an app called Reduce Photo Size for situations where I want to upload photos on the trail or in a trail town and don't have a particularly fast internet connection of whatever sort.
If your RAM space is limited, I suggest apps such as DiskUsage and App 2 SD (I use both of them on my newer, more limited phone).
Apologies if few of the above relate to the sort of backcountry use you're interested in.