In reply to Toerag:
Windows 7 has Live Gallery that will allow you to add tags to the metadata. However it is a very slow process.
If you have a lot of images, possibly in the thousands or tens of thousands, then I most definitely recommend paying for a copy of Adobe Lightroom. Lightroom allows you to batch tag multiple images at a time.
I initially got a pirate copy but it was many generations old so I went and bought version 5 and never regretted the investment.
If you are still after a free option then you could consider using a Linux operating system and then use a photography software that can create tags.
If you are a windows user then Lightroom is the most efficient option.