Windows/Mac/Linux: Free, open source application BananaSplit divides DivX or Xvid AVI videos into user-defined chunks. There are plenty of reasons you might want to split a video using this app, but the two most obvious that come to mind are to highlight a small section of a long video or to share a large video over the internet when you're limited by filesize constraints. BananaSplit is free, cross-platform, requires Java. For a quick howto, head over to Simplehelp's step-by-step tutorial for BananaSplit.
BananaSplitter [via Simplehelp]