I've heard good things about the old Signets electrostats, but I have nevr actually listened to them. I personally prefer the Koss Pro 4aa headphones for mastering work, but they weigh a ton. I like the Stax electrostat headphones, but when they get old, the ribbon material dries out in them, and then they'll crack if you crank em' up one day.
For the modern day studio monitors, I like the Sony MDR-7506 Studio Monitors, they have a rich and full sound, and have 40mm drivers in them. The noise cancellation isn't as good as the Koss ones, but for their price, they sound almost as good as the Pro 4aa's (the Koss ones are just a hair crisper in the high end, but it is really hard to notice). Not only that, but you can still get OEM parts for the Sony's
Jason