Glass floors - as a seamless sheet and includes matching ceiling - PLUS DAYLIGHT by Inge
These look more like a sheet of glass such as you might get in a conservatory. The underside therefore is higher than a normal plaster ceiling, and you will definitely need to use a custom ceiling trim to go around the glazed area, unless you have glazed the entire roof or floor right up to the walls. You may need moveobjects cheat to place over certain areas.
And here is one that lets daylight in like a window: http://www.drealm.info/simlogical/sl/Sims3/GlassFloors/ij_glassfloorseamless1x1WithDaylight.zip
(You can have them both as they are seperate objects.)
Links to suitable ceiling trims can be found in this thread
Angela from TSR has kindly made these into Sims3Packs for those who want to offer glass floors in lots they share:
Angela has also prepared a tutorial to help you make your own: http://www.drealm.info/simlogical/sl/Sims3/GlassFloors/Recolourable Glass Floor, the How To.pdf
For creators who wish to clone these items, I have seperated the floor and ceiling into two packages which makes it easier to clone them or use in TSRW http://www.drealm.info/simlogical/sl/Sims3/GlassFloors/GlassFloorAndCeilingSeperate.zip