> Key your locks alike, then you only need one key!
Seconded!
If you aren't up to rekeying locks yourself it is an easy job for a locksmith and shouldn't be too expensive.
If rekeying isn't sufficient because your locks use different shaped keys and so you'll have to get at least some new locks to put everything on a common key, and you are up to doing lock replacements yourself (which unless you've got unusual locks is usually one of the easiest DIY projects), I'd consider using Kwikset SmartKey for the new locks.
Despite what the name suggests these are not electronic locks. They are entirely mechanical, using a normal key. They came out in 2007 before "Smart" had become associated with shoving microprocessors into places they don't belong.
If you want to rekey a SmartKey lock you simply take the current key, insert it, rotate 90° clockwise, insert a tool Kwikset provides through a little hole in the front to press a button, and that releases the key so you can remove it without having to rotate the cylinder back to 0°. Then you can put in a different key, rotate the cylinder 180° counter-clockwise, then back to 0° and remove that key.
The lock is now rekeyed to that second key.
So, just buy your news locks from Home Depot or Lowe's without having to worry about getting locks that are keyed to the same key, install them, look at all the keys they came with and pick which one you want to be the common key, and then go around and rekey them all to that using the procedure described above.
Keep the other keys. They can be useful if you have a guest stay over (assuming you have more than one door to your house). Rekey one of the doors to one of those other keys and give that to the guest. When they leave you can rekey back to your common key.
Kwikset also makes SmartKey padlocks if you want to go all in on the one key thing.