A while ago, I published a post about expanding the ESP8266 ADC capacities with the PCF8591 8 bits I2C ADC multiplexer. 8 Bits is really enough for most work I would guess, but if you need more accuracy, say 18 bits for a high precision Digital Voltmeter, there is the (relatively expensive) MCP3421. I say ‘relatively coz it is still cheaper (and easier to use) than the LTC2400 (controlled by SPI). This is only a one channel chip, so if you want more channels, you would think you have to get more (which is OK as generally they are sold in webstores in a lot of say 5 or 10), but the chip has no address selection. It has a fixed address of 0x68.
If you do not like soldering though, there is also an ‘evaluation module’ available.
The MCP3421 has an in-program sample rate selection from 12-18 bits and in inbuild Programmable Gain Amplifier that can also be controlled from your program. There is a library available, several in fact which makes using the chip very easy. If one channel is not enough, try the MCP3424 18-bit, 4-channel, multi-address ADC