What is a popular cheap and robust substitute for 2n7000 MOSFET for use with 3.3 Volt designs?
The UM6K1N / 2SK3018 from Rohm work very well with 3.3V circuits (max 1.5V \$V_{GS(th)}\$) and are rated 100mA \$I_D\$ and 30V \$V_{DS}\$. They also have gate protection clamping diodes.

The only downside is that they're surface-mount only - you'll need to spin a small PCB to use them.