Adding motion blur to video or 3D renders

In a way similar vein as creating slow motion with still frames using AE Time Warp or RE:Vision Effects Twixtor Pro (as shown by Eran Stern recently), you can add motion blur to video or 3D renders with AE's CC Force Motion Blur or Time Warp filters or with ReelSmart Motion Blur. Both of the latter 2 filters use motion estimation for better results, as noted in After Effects Help:

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Note:
This post was updated on PVC in October 2011; see Adding motion blur in After Effects.