Most decisions as to what to carry in your camera bag come down to two main factors: cost and space. I recently wrote about DSLR.bot, which I now use all the time when I need a remote shutter release or intervalometer for my DSLR. It saved me from buying another $99+ piece of hardware and I enjoyed the DIY part of the project. While not perfect, it has worked wonderfully and is always with me.
I am also currently testing a light meter app to replace mechanical or digital meters. The app is Pocket Light Meter and the ad-supported version is free. You can find their official website here.
It is important to note that this is a reflective meter only, but I have found the interface to be simple and very easy to understand. It has a spot-meter function allowing you to tap on your iPhone screen to meter different segments of your frame. This is nice for taking multiple readings to look for light fall off. It also allows you to lock in ISO, aperture, and shutter when composing a shot, hence compensating for other environmental or artistic elements.
Pocket Light Meter reads in thirds for shutter, aperture, and ISO but you can adjust it to halve or full stops to work with your preference. You can also add exposure correction from -3 to +3. You'll have to work within ISO 0.8-102,400 and aperture range: f/1 - f/51, but I think you can manage.
One added feature I find useful is the log. The log captures a small thumbnail of your image and some basic data that could replace notebooks for some photographers--especially those shooting film. It will also link to your Dropbox for easy reference once you are back in the studio or on a computer.
Paying nothing compared to a cost of $150 - $650 for digital light meters (granted they are feature rich) means there is no risk to trying this app. Plus I think you will find it to be a really fun tool. Use it to either fine tune or practice your natural skill set or as a basic/back-up light meter. For more information check out this great discussion thread on the topic of software vs. hardware light meters which also covers some of the benefits, limitations, and results of Pocket Light Meter.
Suggested Donation: $4.99 (The developer is British, so buy him a pint)
Pro: Affordable, accurate, nice logging features to replace notebook
Con: Reflective metering only, experimentation required based on your personal gear set-up