As a relative newbie to electronics, I probably don't have the chops to go up against many of the amazing projects already listed. But whenever I talk to people who actually do know what they are doing, I hear the same set of comments about how the logger would be vastly more efficient if I went to a custom PCB, with a pic micro controller, or that if I used a 3d printer for the housing it would look more professional. And that's all great advice if you are trying to bring something to market, but not so good if you want something that you ( and others ) can really tinker with.
In the spirit of Sheckley's 'Minimum Man', I wanted something with lowest common denominator flexibility: so that modifications and part replacements don't require a significant re-write of the operating code. This means that you can build one with whatever components you can get your hands on, and I'm betting that even with $2 eBay modules I will reach that one year 'good enough' point for serious research.
This includes the tools needed for the build: I have worked hard to come up with a robust underwater enclosure that can be assembled with basic tools (ie: a drill, a hack saw & sandpaper) A good housing is as important as the electronics because environments that are challenging to observe, such as those underwater and underground, are poorly monitored although our society depends on them for vital resources such as ground water. This is especially true in the developing world where even the cheapest commercial equipment that can withstand these conditions is out of reach: