Host or hostess gifts are a way to thank the host(ess) for their effort (and money) in hosting a party or a dinner. A good rule of thumb is to match your gifts "effort" to the effort they put into their party. Usually if you're bringing a dish you should not be supplying a host(ess) gift although gifts are always appreciated.
Ask yourself a few questions to get your bearings
What is the size of the party/get together. A larger party requires more work and a nicer gift would be in order... Has the host(ess) spent a lot of money or put a ton of work into it? If so maybe you should too... How well do you know the person hosting? Is their something you know they like (golf, games, wine, cooking).
Consider bringing some Alcohol
A nice bottle of wine for a holiday party (try "Layer Cake") Or beer for a barbecue. Alcohol in any form can get expensive when buying for many people. To provide some for the masses is a gift in itself.
Flowers seem to be something people just don't use anymore. Pick up some fresh flowers (in or without a vase) and the host(ess) can put them in the middle of their table or to brighten up their room. Always an acceptable gift so long as they are tastefully chosen (don't reflect funerals such as white roses).
Help with the Entertainment
Think appetizer plates, wine cork decorators, cookbooks, napkin rings, glass rings (the kind that people use to tell their drinks apart). Cocktail napkins.
Bring some nice bon-bons or chocolates.
Or some gourmet coffee, tea, or even hot chocolate
If all else fails buy a gift certificate to a local store (like a bookstore or a coffee shop) and fold them into a handwritten thank you note.