Common digital "glue" chips used with Arduino/Netduino?
OK, this is what I wound up ordering from two vendors. This isn't a plug;  these are just two that I usually order from.

From SparkFun:


BOB-10402 EM-406 Connector Breakout 
COM-00312 Darlington Driver 8-Channel ULN2803 DIP
DEV-07914 Arduino ProtoShield Kit 
DEV-10059 Arduino and Breadboard Holder
GPS-09123 Interface Cable for EM401 and EM406
PRT-09279 Arduino Stackable Header - 8 Pin 
PRT-09389 Jumper Wires Premium 12" F/F Pack of 10
PRT-10158 Break Away Headers - Long


From Mouser:


DS1822+PAR Board Mount Temperature Sensors 1-Wire Digital Thermometer
SA612AD Up-Down Converters DOUBLE BAL MIXER/OSCILLATOR
74HCT4066N Analog Switch ICs QUAD BILATERAL SWITCH
MAX232ECNE4 RS-232 Interface IC RS232 Line Driver/Receiver
122-0421-GR BAT HLDR 2XAAA 6 LDS NKL PLTD 6 LDS 26AWG
AB38T-32.768KHZ Crystals 32.768KHz
Desc.: Board to Board / Mezzanine Connectors HDR VT 1X09P .1 230/110 30AU
SN74LS247N Encoders, Decoders, Multiplexers &amp; Demultiplexers BCD to 7 Segment
SN74LS595N Counter Shift Registers Serial-in shift Register
DS1307+ Real Time Clock 64x8 Serial I2C RTC


The SA612 seems like an odd choice for general tinkering;  I'm a ham radio operator, though.  I did decide to get the temperature sensor; we have a 600 dollar IT environment monitor at work and it is tempting to replicate that with a 59 dollar dev board for my personal server.  The battery holders are related to the RTC I'm planning to use, and I've gotten an assortment of 74LS logic so I can interface an LCD (Hitachi-based) and some seven-segment displays I have.  The GPS parts are for the specific module (EM406A) I got from SparkFun.  

To broaden this for other experimenters, I'd also recommend the following things I have in my parts bins:


Small-signal NPN and PNP bipolar transistors--get bunches of each
small-signal diodes of the 1N4148 variety
for radio experimenting, 1N34 germanium diodes if they even still sell them
1N4001 small switching diodes
Zener diodes of 5.1V, 7.1V etc.
LM7805, 7812 regulators.
LED's (naw really!)
breadboards, protoboards and jumper sets


The Maker Shed has a nice starter kit for $120 with an option to get it with your choice of Arduino, Netdunio or Netduino Plus, which covers many of the items on my list.