What's a French Dip?

Operational Definition 
A French dip is a sandwich composed of bread and at least one other component--typically close-faced--that is dipped into a hot broth or gravy.  It doesn't have to be beef and there are examples of vegetarian French dips.  The classic French dip is a French baguette with roast beef dipped into pan drippings.  The dipping can occur either by the eater or before serving (served "wet").

See the Wikipedia page about French dips.

Breads 
Ciabatta
French baguette
Giuseppe (what is this?  Grim's in Seattle has it with their dip...)

Meats (or the main protein-like substance)
Hamburger
Roast beef (can be served rare for style and taste/flavor like at Grim's Seattle)
Prime rib
Ham (Turf, Seattle)
Lamb (recipe here)
Turkey
Portobello Mushroom(s) (like from Cafe Flora, Seattle)

Non-Meat Toppings (Accompaniments)
Cheese
Swiss
Provolone
Mustard
Havarti

Other
Mushrooms
Grilled onions
Carmelized onions
Smoked Onions (Redmond's Bar & Grill, Redmond, WA)
Horseradish
Horseradish Aioli
French Fries (Tat's Deli, Seattle)

Au Jus
Beef drippings
Gravy
Ancho Chile Au Jus (Peso's Seattle)