Apple - How do I force iPhoto to download new photos in my Photo Stream?

I have found a better way. One that doesn't involve resync'ing the entire stream.

1. Quit iPhoto
2. Open up Activity Monitor
3. In the search field enter: photostreamagent
4. Select all the PhotoStreamAgent processes listed
5. Click the Quit Process button
6. In the confirmation dialog that pops up click the Quit button
7. Confirm that all the PhotoStreamAgent processes are no longer alive by waiting for the Activity Monitor window to show no processes with that name
8. Re-start iPhoto

When you restart iPhoto it will re-start the PhotoStreamAgent process for your iPhoto session and kick off a new Photo Stream sync. You'll get your lastest pictures updated and you won't have to sit through a full re-sync of all your Photo Stream.

Update for Yosemite where the process name has changed to iCloud Photos. This is the what you want to kill:

Update for Yosemite 10.10.4 where the process name has changed to Photos Agent. This what you want to kill:

You can also do this, quickly, from the command line like so:

sudo pkill cloudphotosd
sudo killall cloudphotosd

In iPhoto, turn Photo Stream off then on.

Go to Preferences > Photo Stream. Uncheck "Enable Photo Stream". Close the preferences window. Click Photo Stream in the left-hand navigation pane and confirm there are no photos. Turn Photo Stream back on in preferences (check "Enable Photo Stream"). After a few seconds, iPhoto will start to download all the photos again. Depending on the number of photos, and your bandwidth, it may take awhile.