PLEASE NOTE:
SyncPhotos is not compatible with Apple's new Photos app
Requires Mac OS X 10.10 ("Yosemite"), 10.9, 10.8 or 10.7
and iPhoto '11, iPhoto '09 or iPhoto '08
More info: SyncPhotos Release Notes SyncPhotos Manual
Not sure which photos and videos are on which computer?
Launch SyncPhotos on 2 Macs and you'll see which photos and videos the other Mac has that you don't already have:

"After you try it, you will definitely want to continue to use [SyncPhotos]." - Macgasm
"[SyncPhotos] Lets You Effortlessly Transfer Photos Between iPhoto Libraries" - MacStories
"The Easy Way To Sync iPhoto To Another Mac" - Mac360
Browse the photos and videos and retrieve the ones you want. SyncPhotos copies them from the other computer and imports them into iPhoto.
If you want to receive photos and videos from another computer without having to go check for them, check "Automatically import":

Choose to import all new photos and videos, or only from albums you choose.
In automatic mode, SyncPhotos's "agent" program monitors the other computer and automatically retrieves new photos and videos as they become available. You can choose to import all new photos and videos, or only the new photos and videos in selected albums.
SyncPhotos is ideal for households with shared cameras but separate computers. No more asking whether the other person got the photos from the camera before you clear the memory card!
SyncPhotos can retrieve photos from Computer A to Computer B, or Computer B to Computer A, or both!
SyncPhotos only retrieves original photos and videos from other computers you specify. It never deletes anything from your iPhoto Library, and it never overwrites your modifications. So it's very safe.
Automatic mode supports retrieving photos and videos from only the albums you choose, which makes new workflows possible. For instance, you could select photos and put them in an album, and set up the other person to retrieve photos only from that album.
Configure SyncPhotos on a Mac Mini to automatically retrieve all new photos and videos from all the computers in your family. Each member of your family can browse the Mac Mini's new photos and videos and retrieve the ones they want. Or they can choose to automatically get new photos.
iPhoto's built-in sharing feature lets you view photos in another computer's iPhoto Library, and even copy them to your own iPhoto Library, but:
NOTE: SyncPhotos copies Events, but it does not copy titles, Albums or other metadata. See the FAQ for more details.
Check the FAQ, read the manual, or send email to support@haystacksoftware.com.
My wife and I each have a MacBook Pro, but we share our 2 cameras for taking family photos. We both want the photos, but sometimes we forget to import from the cameras into both computers. I always got the feeling I was missing some photos that she had, and vice versa. When I first used SyncPhotos, I found out she had 4,100 photos that I didn't have, and I had 7,000 photos that she didn't have!
 
Now we both have all the photos, and whenever one of us adds photos to iPhoto from a camera, the other person gets the photos automatically. If one of us adds photos to iPhoto while traveling, as soon as both computers are on the same local network again the other person gets the photos automatically!
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