A bunch of diodes 1N4004 or 1N4007, use for coils or switches. ( 1N4001 are normally on switches but lower voltage, I don't use them as the others work fine and I'll never accidentally mix them up and get on a coil and blow things up ).

Diode, Rectifier, 400V, 6A For newer driver boards

Bridge Rectifier, 400 Volt, 35 Amp, Wire Leads these guys for earlier games.

LTG : )

Clay says to always have this stuff on hand:

#44 light bulbs: have 20 or so around. Fifty is plenty to do most games. Many people suggest using #47 bulbs instead, as they consume less power and produce less heat, but are also less bright. As a compromise, I personally like #44 bulbs for the computer controlled lights, and #47 bulbs for the general illumination lights. Note "import" 44/47 bulbs are fine and are less expensive (I don't personally see the need to spend more money for the General Electric bulbs).
#555 light bulbs: have 20 or so around. Fifty is plenty to do most games.
#906 or 912 flash bulbs: have 10 or so around.
#89 flash bulbs: have 10 or so around.
#86 bulbs: used in Twilight Zone and Creature from the Black Lagoon only.
#455 or #545 bulbs: blinking style bulbs. The #545 have a (#555 style) wedge base. Used only in Twilight Zone, Addams Family and No Good Gofers.
Fuses: I would have five of any needed value on hand at all times. The voltage rating really does not matter for fuses, as long as they are at least 125 volts. Coils run at 50 volts, only the line voltage fuse in European games could be higher than 125 volts. The fuse voltage only determines *how* the fuse blows (it's the current rating that determines *when* a fuse blows). Do not buy 32 volt fuses.
WPC-S and Earlier Games:
For WPC/WPC-S games, these use 1.25" long, 3AG fuses. Radio Shack sells fuses for a decent price. Slow-blo fuses are known as MDL fuses. Fast-blo fuses are known as AGC fuses. At minimum have:
3/8 amp fast-blo (for dot matrix display)
3/4 amp fast-blo (used for 12 volts)
2 1/2 amp slo-blo (used for flippers on non-Fliptronic games)
3 amp slo-blo (used for solenoids, 12 volts, flippers)
5 amp slo-blo (used for general illumination, flash lamps, solenoids, +5 volts)
7 amp slo-blo (for solenoid voltages)
8 amp fast-blo (used for playfield lamps and line filter)
WPC-95 Games
Uses the new European standard, smaller GMC, GMD or GDC "T" fuses ("T" means timed, aka slow-blo). All 250 volts (but 125 volt fuses work fine too). WPC-95 uses only "T" (GMC, GMD, GDC) slow-blow 5x20mm fuses. These fuses are smaller, about .75" long. Note these are not GDA, GDB or GMA "F" (F means "fast blow") fuses. Buss uses the "GMC" and Littlefuse uses the "T" terminology ("T" and "GMC" are the same thing). Note these fuses are small and difficult to tell if they are fast or slow blow. So look for the "T" impression on the metal fuse ends to confirm they are slow blow fuses (Radio Shack often mistakenly calls the 5x20mm slow-blow style fuses "GMA" F instead of "GMC" T). Have available:
T0.315 amp (audio/video board)
T0.63 amp (driver board)
T2.5 amp (audio/video board)
T4.0 amp (driver board)
T5.0 amp (driver board, line fuse)
T6.3 amp (driver board)
Nylon Coil Sleeves: the longer 2 3/16" length (part number 03-7066-5) are used when rebuilding flippers. The 1.75" length (part number 03-7066) are used for pop bumpers, etc. Sleeves with a lip (part number 03-7067-5) and tubing on each side (known as an "inline" sleeve) are used on the knocker, etc.
Flipper Plunger/Link: used when rebuilding flippers (part number A-15847 or A-10656).
Flipper Link Spacer Bushings: these small bushings go inside the flipper links (part number 02-4676).
Flipper Coil Stops: used when rebuilding flippers (part number A-12390).
Flipper EOS Switch: part number 03-7811 (non-Fliptronics) and SW1A-193 (1992 and later Fliptronics games).
1/4" Heat Shrink Tubing: this is used on the flipper pawl when rebuilding flippers.
Blue Spring Steel: used for rebuilding the entrance of clear plastic ramps. Available at http://www.mcmaster.com, order the .006" thickness.
Shooter Spring: the short chrome spring on the outside of the shooter mechanism (part number 10-149). These rust and look like crap in short order.
1 1/16" Pinballs: a new pinball will make a playfield last longer.
Leg Levelers: replace those old crummy looking leg levelers with brand new ones. 3" are used on solid state games.
Rubber Rings: order game-specific ring kits with exactly the rings needed (from Pinball Resource). Don't forget to get flipper rubbers and a shooter tip.
Transistors:
TIP102 (coil drivers)
TIP107 (lamp matrix)
2N5401 (coil pre-driver, score display)
2N4403 (fliptronics)
2N5551 (score display)
2N3904 (score display)
TIP36c (high power coil driver)
MJE15031 (score display)
MJE15030 (score display)
LM323 (driver board WPC89/WPC-S)
Diodes:
1N4004 (flipper coils, switches, lamps)
1N4148 or 1N914 (cpu board)
P600D or 6A4 (WPC95 power rectifying)
1N4742 (12 volt, score display)
1N4758 (56 volt, score display)
1N4759 (62 volt, score display)
Resistors:
1.8k ohms, 5 watts (score display)
4.7k ohms, 5 watts (score display)
120 ohm, 5 watts (score display)
120 ohm 1/2 watt (score display)
47k ohms 1/2 watt (score display)
39k ohm 2 watt (alpha-numeric score display)
22 ohm 1/2 watt (driver board)
Chips:
ULN2803 (cpu board switch matrix, driver board lamp matrix)
LM339 (opto voltage comparitor for opto board/cpu board)
74LS240 (cpu board)
74LS244 (cpu board)
74LS374 (cpu board, driver board)
Bridge Rectifiers: for WPC-S or earlier game, keep a few 35 amp, 200 volt (or higher) bridge rectifiers around, with wire leads. The industry part number is MB3502W (Williams part number 5100-09690, Mouser part number 625-GBPC3502W, Digikey part number MB352WMS-ND).
Connector pins and housings: used to repair burnt connectors. Buy the plastic connector housing (12 pins Molex #26-03-4121), the board header pins (12 pin Molex #26-48-1125), and crimp-on .156" Trifurcon terminal pins (three wipers), part# 08-52-0113 (tin plated phosphor bronze) for 18 to 20 guage wire (part# 08-52-0125 for 22-26 gauge wire). Tin plated phosphor bronze is the best pin material, as it has better spring, fatigue resistance and current capacity. But if this part number is not available, part# 08-50-0189 (tin plated brass) can be used instead.
Optics. LED transmitter optics and "U" shaped optics are good to have on hand. Radio Shack sells the infrared LED (transmitter), part number 276-143C, $1.69 (replaces Williams A-14231). The "U" shaped optos QVE11233.0086 (as used on flipper boards) are available from Competitive Products (remember there are several types of U shaped optos).
15,000 mfd 25 volt "snap caps". These capacitors are used for rectifying +5 and +12 volts. Digikey part# P6577-ND, $5.52 each.
470 mfd 25 volt electrolytic cap for driver board C4.
Triacs: used for the general illumination circuit (not needed very often). The specs for a WPC triac to use are pretty lose. For example all these work: BT138-600E, BTA12-600, NTE5671 (800v 16amp), NTE56010 (800v 15amp), or NTE56008 (600v 15amp).

Newer Stern games use the IRL540 driver MOSFET for coils, instead of bipolar transistors. I don't know if these ever go out, but it might not hurt to have a couple in the kit.