Try Raclette. If you can find it, it will be well worth it. I use it for making cheese smother scallops, but it goes great on all kinds of things. It is almost impossible to find. Some Von's carry it, and some other high end grocery stores and wine and cheese shops.

It's a swiss/french cheese(must be from the Alps) and is a melting cheese that people eat melted with bread or crackers, it also goes very well on scallops and many other foods. It has a relatively strong flavor and costs about the same as Gruyere, although the texture is different. The raclette will be softer, by a little bit.
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It depends on how much cheese the recipe calls for, and how integral cheese is to the texture of the finished dish.

Fontina might also be good, which is an Italian cheese and a good melter. The younger, not the aged version. It's a lot milder than Swiss or raclette.

If you don't mind being somewhat blasphemous, you could use a sharp cheddar. Again, depends on if the cheese is essential to the texture of the dish or is mostly there for flavor.
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The most easily-found choice in the USA might be Jack. Plus, there are often variations of jack cheeses with peppers, etc, nice for recipes.
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http://en.wikipedia.org/wiki/Vacherin_(cheese)

Vacherin sounds like a comparable cheese. I would also guess Baby Gouda would work but I've not worked with it alot.
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Any mild swiss cheese would make a good substitute. Gruyere is a semi-hard cheese, that's a bit stronger than your standard swiss cheese, but regular swiss works just fine in most recipes.
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I use Swiss cheese, or a combo of Swiss and parmesan.
Source(s):
Experience

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