There is this new tool out there to help document a garden. My wife and I have used spreadsheets, word documents and notebooks to keep track of things. She does more with plants, I do more with hardscapes and other projects (ponds, stones, mulch, woodshed). This Muddy Boots tool allows us to keep records and photos and journal notes in a cloud application and access it on our laptops and smartphones. It's integrated with these QR-coded plant tags. So you can scan a tag and see all the information about one of your plants (including pictures and history) on the phone itself, rather than trying to read a tag that's been written on or etched. Pretty cool.
Here's a Fothergilla in our garden that has one of those QR tags on it. It turns out you can actually scan this photo and see the plant info - I just tried it and it worked.