Basics tools like a good hammer, measuring tape, cordless drill or good circular saw are essential even if your builder is just a weekend carpenter.  For basic home repairs and doing outdoor jobs like fixing the porch step or repairing a fence all simply require a few tools like these.  When choosing a gift like a cordless drill keep in mind that if he has a big job to finish,  a larger more powerful drill can really get heavy as the battery size increases with the power.   You don't want him quitting half way through because his arm gets tired!
 

If he enjoys woodworking, consider a fine set of woodworking tools.  Fine chisels, planers, small knives and sanders are all great gifts for the woodworker.  Often a hardward store or hobby shop can recommend a few basics if you want to get him a few 'starter' tools as well.  Electric sanders are great for all kinds of refinishing projects.  Today there are many types of sanders all with different shapes designed for specific jobs and surfaces.
 

If you have lots of home repair jobs or you are just starting out,  then consider getting a few basic home tools,  like a good wrench, blowtorch, (and fire extinguisher!), hammer, drill which can also be used as an electric screwdriver, drill bits, measuring tape, a good assortment of nails and screws and don't forget a first aid kit too!
 

You might even consider a major gift like a table saw, table router, drill press, or other big tool item for his shop.