Frankly, none of the options mentioned here are as reliable as SMS. In many instances, I have received whatsapp message hours after it was sent (both connected to the internet during all that period.)
You can send free SMS using AOL messenger as well. My only issue is, how can you intercept the SMS sent to you before it is recorded on carrier's network. In my case, one of my colleague sends me a ton of messages from AOL messenger instead of a call and that is sucking up my credit like crazy. I would like to intercept all those messages and receive them for free. I could give him google voice number to send SMS on but I would like to know if there is any other option.