At some point your fencing weapon will break, and then you'll need to either fix it, buy a new one, or get someone else to repair it for you. The most cost-effective thing to do is to be able to perform at least minor repairs for yourself. Most repairs in foil and epee involve tightening the grips and replacing springs in the tip, but you can end up rewiring the electric blades as well.
Here are a few suggestions for tools that you should keep in your fencing bag to perform some of the regular maintenance and testing.
Every fencer needs a small toolkit for equipment maintenance. The following tools and supplies are essential:
precision screwdrivers for point maintenance and assembly; also handy for body wire repair.
pliers for tightening points; wire cutters are also useful, and are incorporated into many pliers.
Allen wrench, screwdriver, or socket wrench for pommel nuts.
quick-drying (eg. cyanoacrylate) glue for emergency wire repairs.
cloth tape for insulating foil tips.
Fencers who do a lot of maintenance will also find the following tools useful:
metal file for fitting tangs into guards/grips.
hacksaw for cutting tangs down.
blade-bowing tool (see 2.12) for gluing.
scraping tool for cleaning old glue out of grooves; an old jeweller's screwdriver will do, provided you don't mind ruining it. Utility knives will also work.
Lighter for burning off wire insulation or softening springs.
vice-grip pliers for heavy-duty work away from a work bench.
Swiss-army knife for everything else.
weapon-tester box.
Serious armourers will need many other tools, including:
workbench with vise.
ohmmeter or multimeter Heavy Duty Electric Soldering Irons .
mask tester.
metallic fabric tester.
body wire tester.
set of weights and shims.
soldering iron (light for wires; heavy duty for pistes).
Dremel tool.
A common additional tool suggested by some on the Fencing.Net forums is a propane torch. (Though the torch may well do more harm than good.)
Craig Harkins runs Fencing.Net - with resources on the sport of fencing and also has a full service site selling fencing gear including blades, uniforms and other equipment for beginning to advanced competitors.