Like everyone else said, it depends on far too many circumstances to fit into one simple number. It depends on other expenses, income level, interest rate, etc. However, if you just want a very crude estimate of what you can get (before you talk to a mortgage broker for a real estimate), 3X is a good conservative starting point today.
Most people now are approved at a value near 4X. Over the last couple of years, mortgage money has been too easy to get and 5X or higher wasn't too uncommon. Of course, foreclosures are now at an all time high (so while you may be approved for the money, it might not be a good idea). I like to be more conservative, yes you can get approved and probably afford 4X now, you probably might not enjoy that life. Go slightly cheaper (closer to 3X) and you'll have plenty of money for remodelling, for vacations, for furniture, for entertainment, etc. I don't like to encourage people to be house-poor.
Ameesh's number of 50% is another similar benchmark. For years people tried to keep that at 30% and they did just fine. Recently mortgages were too easy to get and people often used 50% or more. But then, like Child of Wonder pointed out, that doesn't leave much if any to actually enjoy life. Life is about enjoyment, not about getting as much house as you can possibly get away with.
But then again, your circumstances may vary and maybe you can afford far more than those two crude estimates.