On the model B+ there are four screws that can be used to attach your device either to a base or firmly inside of a case or other enclosure.
What is the size of these screws? That is, if you are walking down the isle of a hardware store, what numbers should you be looking for?
tl;dr: Those models that have holes in the PCB are made for M2.5 (or UNC 3-48) screws and a length fitting to the enclosure or respective counterpart.
Pi 1 B+ contains four M2.5 mounting holes - supposedly drilled to 2.75 +/- 0.05 mm. This information is taken from the "Raspberry Pi B+ Mechanical Schematic" (official drawing by J. Adams, 07/03/2014, to be found at RS Components).
Pi 1 B (rev 2) contains two M2.5 mounting holes (as derived from own measurements, diameter is roughly 2.8 mm). See also http://www.raspberrypi.org/upcoming-board-revision/ that describes the existance of drilled holes of 2.9 mm diameter for M2.5 screws.
Pi 1 B (rev 1) does not have any mounting holes.
Pi 1 A shares the board layout with the Pi 1 B (suggested by TomG and supported by available pictures).
Addendum (per jmb's comment, thanks!):
https://www.raspberrypi.org/documentation/computers/raspberry-pi.html has now mechanical drawings for the Pi 4B, Pi 3B+, Pi 3B, Pi 3A+, the Zero (v1.2), the Zero W, Pi 1B+, Pi A+ available. They show the size and position of the holes as well as board outline and position of connector. These models all share the 2.75 mm (or 2.7 mm respectively) holes.
I've just had a look at the Raspberry Pi A/B/A+/B+/2 PCB mounting holes and they all seem to be a little over 2.5mm diameter so a M2.5 machine screw/bolt would be the best fit. I usually use Nylon M2.5 bolts, washers, spacers and nuts when mounting boards to the Raspberry Pi.
Just to add to the already correct answers, there's enough leeway to enlarge the PCB holes to M3 size so that you can use standard PC floppy drive screws, of which any PC enthusiast is likely to have a surplus. These often fit into the brass motherboard stand-offs that come with PC cases (and are readily available online) so it makes for a cheap and easy way to securely mount the Pi.
Just remember to use a drill to enlarge the holes, as forcing an M3 screw through the original sized hole makes cracking sounds and so you risk snapping the PCB that way.
You also need to avoid the older screw design that has a washer molded into the screw head, as this makes the screw head diameter too large and it scrapes against surrounding components like the micro USB connector, while being screwed in. Newer screw designs (~mid 2000s) seem less likely to have this shape so aren't a problem.
Here's a photo of a Pi mounted with floppy drive screws into motherboard stand-offs, which are themselves screwed into a plastic case (after drilling 2.5mm holes into the plastic to ensure a tight fit for the standoffs).
I use (non metric) size 4-40 x 1/4" nylon screws. If they are a bit tight, the nylon gives without damaging the circuit board holes. I seat them into standoffs of the same thread.
