I use a spreadsheet. MS Excel but Open Office is completely free and would work almost as well. Just create fields for these items...name of plant, where purchased, price, annual notes, etc.

There is some company that provides software to the likes of the RHS, BBG, Longwood etc. for plant tracking. I forget their name but I've seen their website before. Most of those programs, like the ones used by libraries to catalog their books, run on a commercial RDBMS although that is completely transparent to the end purchasers.

BTW, as for the mapping part of it, I've seen a program for doing that but I can't remember what its name was. Maybe you should ask in a forum with more traffic. I have a field in my spreadsheet for "location code" but I haven't gone so far as to actually develop a coding system for my yard, yet.