home depot sells a screwdriver with an orange handle, by buck bros i think. it's got 4 bits, 2 flat and 2 phillips - this will take care of almost every screw you come across.
a set of precision (the small ones, usually in a blue plastic box) screwdrivers - good for laptops where there are small screws.
a set of needle nose pliers - standoffs mostly, you can also get nut drivers, but a good pair of needle nose pliers work for more situations.
a pair of tweezers if you have to move around jumpers (or even better a pair of hemostats).
small swiss army knife or a pair of wire cutters to snip zip ties.
small flashlight to see inside the dark spots of a case.
a bag of zip ties (usually just need the short ones, but maybe a few bigger ones for doing up power supply cables and such).
something to put all this stuff in.