I've seen others mention Sony Vegas or Adobe Premiere...are there others software that can do this easily?
Not that I know of, however there are a couple of ways you can do slo-mo depending on what NLE app you are utilizing, for a Mac user in FCP you would use Cinema Tools and do a conform of 60FPS to 24.
Another method for the PC user is with Sony Vegas where the frame rate is not converted but the timeline is stretched, to do so, drop the video to be edited into the timeline, then hover the curser over the right most part of the video to be slo-moed, and while holding down the control key left click on the video and drag it to the right stretching it into the timeline.
Now obviously, both methods (Mac and PC) also stretch the audio timeline, as such you will need to re-dub your audio once editing is complete, and before rendering.
There's a video speed adjustment as well as a slow-motion video setting (.8X to .25X) with frame interpolation.
I think the point of the OPs request is to AVOID frame interpolation.
Exactly. I've searched for this option myself, and I'm surprised at how very few consumer-oriented applications are geared toward it. With all the new DSLRs that feature 720p at 60p, you'd think there would be a good/cheap solution by now to take that 60p and turn it into 24p (40% speed) really easily.