A Low cost A/D with a Potential of 5KV Isolation from Computing elements by use of Normal/Special Opto-Couplers. This can give better resolution and accuracy than a 8 bit A/D. may be close to 10 bit A/D. Added advantage is that you can optically isolate the inputs.
Suppose your temperature sensor touches a heating element OR your speed sensor is subjected to a shock from a defective motor with earth line broken. Such situations happen in the field, even at home or lab. Sometimes it is carelessness, othertimes it is wear-tear. Manytimes it is breakdowns, accidents etc. So do not let the High voltage to get at your Arduino or PICAXE.
More about Embedded Micro like PIC, AVR can be found here – Hyper Embedded