I've tried all kinds of software and I'm still surprised that my hands down favorite is Microsoft Office Picture Manager (came with MS Office 2003).
Picture Manager was about the only softare I found that could reduce my pics by any percentage and still keep the quality perfect. I may use other software to doctor and enhance pics, but Picture Manger is always my "go to" software for reducing pics.
As mentioned above, "do you want to reduce one pic at a time or a whole group?" Picture Manager let's you do one pic or a whole bunch. I've done as many as 300 reductions in one shot and it always works out perfectly. You can choose custom sizes or choose from standard reduction sizes.
Curious what others have had success with though. Lots of stuff out there.