RemOs - see what they've done with the name there? - are, very simply, 'o'-rings made from Mylar (the same stuff that the company uses for its heads) which are available in sizes between 10" and 16" to suit most snares and toms.
Super easy to use, the RemOs just sit on the head and provide a very controlled, focussed tone. The 1"-wide rings don't completely kill the tone of the drum by any means, although there's less flexibility here than with other products.
Simple to fit and control.
Could offer more flexibility in terms of tone.
These Mylar rings blend well with Remo's heads made of the same material - a nice, simple product.
Controls drum sound
Removable EQ Muffle Ring