The kinder, gentler file
I have extremely thin and weak nails (thanks parents!) and using the files that come in nail kits or the file attached to my clippers is impossible.  Most files are so rough that they chew right through my nails.  Glass files are a godsend, they're gentle enough that I can shape my nails and get a clean edge, without taking off half the nail.  They're also pretty durable.  You can break them if you really try, but generally they're going to hold up during normal use.  I have mine sitting in my pen cup and have had no problems.
Rating: 5 stars