Some LEGO Technic sets apparently come with instructions to add a motor, using the pieces from the Power Function Accessory Box (set 8293).
Is there a list of the sets which do contain such instructions?
I recently purchased both 42006 Excavator and 9396 Helicopter, both of which comes with instructions on how to add motors.
The reviews for 9395-1: Pick-Up Tow Truck suggest it can be motorized.
A lot of the more recent Technic models are either too small for a motor or already come with power fuctions.
If you are looking for current Technic sets that have the Power Functions option, you can search LEGO.com by the term "power functions" and then filter by "Technic". This will also bring up sets that include Power Functions accessories in the set, so you have to look at the descriptions to find out more. As of the date of this post, the search also returns some retired sets.