I don't know that everyone would necessarily classify them as "tools", but rather essentail to have are some basic emergency / safety equipment, including wheel chocks (I like the rubber triangular ones with handles, such as Harbor Freight sells for reasonable prices), warning triangles, etc. A high-visibility vest is a very good idea, too, should you break down on the road.
Otherwise, I've tended to start with fairly limited tools and figure I can buy any that become necessary while away. Absolutely required, as mentioned by others already, are a #2 squre drive screwdriver (a good, old-fahsioned manual one at the least--there are plenty of places where a power driver is not practical to use), an adjustable wrench or two, maybe a pair of chanel-lock pliers or similar, a simple multimeter, gorilla tape, and the wrenches and jack and such to install the spare--assuming your wheels are of a size that one has some hope of managing. I've also found a small compressor very useful for topping up tires when needed; I have a Fini AirBoss (which is also sold under the Craftsman brand) that I'm very happy with. Needless to say, the compressor must be able to get to the pressure your tires require, and do so without taking hours on end. 12V units that plug into the lighter socket generally don't meet that requirement.