I'll be abroad for a few years and I want to keep my mobile number till I come back. What would be the least expensive way to do this?
You can also port your number to Google Voice for a one time fee of US$20. This will be signficantly cheaper than most other options, and you have the ability to port your number out of Google Voice for free. (If you brought your number into Google Voice, Google waives their US$3 fee.)
The benefit to this is that you can continue to receive text messages that deliver to your Google (Gmail/Google Hangouts) account on that number while you are gone.
Verizon, and probably all the major carriers, offer prepaid sim only deals where $100 will keep your number for a year. At $8.50 or so a month it is probably cheaper any other plan you will find. As you only need to keep a few pennys worth of credit to keep the number, you might be able to resell, at a loss, some of the time by letting friends make calls or surf the web from your account. I am not aware of anyway to suspend your account for long periods of time.
I've got two extra lines, for visiting friends & family. They're used every few months. I keep them on T-Mobile Pay-As-You-Go, which is currently $3/month.
You can use an unlocked VoIP SIP service like Anveo.com, where the cost of keeping a US/Canada phone number on the Personal Unlimited rate plan is 2 USD per month, which includes an unlimited number of incoming voice calls for personal use, plus they even support SMS (most unlocked VoIP providers don't). The normal cost to port a number is 15 USD, but they often run a porting promotion in that they'll do the porting for free if you prepay Personal Unlimited for 12 months, e.g. it'll cost you a total 24 USD for first year, then 2 USD per month thereafter, to have an unlimited use of your phone number for incoming phone calls anywhere in the world for as long as you please. There are also other alternative SIP providers, but most of them don't support SMS.
You can use the service with any software or hardware that supports SIP. E.g., you can buy one of the many hardware phones like Cisco SPA303, or you can you an unlocked softphone on your mobile phone like Acrobits Softphone (or, if you're using Android, it may even have integrated SIP support out of the box directly within the Phone app).