SMS-only to Data-only texting My wife has an unlimited SMS plan, but she doesn't have a data plan or access to WiFi.  I have a data plan, and can receive unlimited SMS but I can't send them for free.  We're both on Android.  Is there a way for us to send/receive messages without me having to buy a text plan or her having to buy a data plan?
If you are located in the USA, Google Voice with Hangouts app may be what you're looking for.

Your wife doesn't need to change anything, as she can text using her carrier's SMS plan.  You will need to do the following:


Register for a new Google Voice number here using your Google account.
Install the Hangouts app on your phone.
Open the Hangouts app, and sign in with the same Google account you used in step 1.
In Hangouts app go to Settings -> SMS, tap on "SMS Disabled" and enable the Hangouts app to be the default SMS app.  This may be different depending on the Android OS version your phone is running.  Refer to this site for more details for pre-4.4 instructions.
In the same Settings screen ensure the "Send SMS From" option is set to "Smart Reply".


Now if you open Hangouts and tap on your wife's contact, a new Conversation screen will open.  On the bottom, to the left of the text entry area, tap on the green text icon and tap "More".  This will bring up all the options, including "... (via SMS)" and "... (via Google Voice)".  The "(via Google Voice)" is the option you want, as it will use your data plan instead of your carrier's SMS plan.

You're all set - your wife will only need to make sure that she texts you at your newly created Google Voice phone number, and not your cell number.  Other than that, she can continue using her SMS plan, while you will be using the data plan, and avoid SMS charges.

Once again, Google Voice is currently only available in the USA, and does not support sending texts to international numbers.