Along with clamps, it seems that one thing you can't have too many of is marking gauges. You start off with a single multi-gauge but you soon come to realise that it's really handy to be able to leave a gauge set for various parts of a project: one set for those hinges, another for the mortises that also run into the dado cuts on the door frame and another for the depth of those dovetails you need to cut - just to be on the safe side. Another thing is that you'll start to need gauges that are different sizes for different jobs. It 's quite hard to mark a 1/8" mark on a small box using a big beefy gauge.
A decent gauge can cost quite a bit though (starting at about £15-20) so it starts to make sense to make your own. You can then tailor your gauge to you. You can also decide on the type of gauge you want. Some people prefer pin cutters (that bit that looks like the tip of a nail sticking out of the wood) whilst others (myself included) like a blade.
This gauge is based on a design that Matt Kenney from Fine Woodworking magazine published. I had to use a nut and bolt instead of a nice thumb screw since a couldn't find anything else to use (and besides, the idea is to save money right?). The main timber is an old American White Ask off-cut that I had and the wedge is made from a bit of Beech (the round bit of wood is taken from a length of standard doweling). Finally the blade itself is made from an old jig-saw blade. I just ground the teeth off and then created a suitable bevel at one end.