Updated to clarify that a split clip feature exists in 10.7's version of QuickTime X.
Say what you will about the QuickTime X framework and player introduced in Mac OS X 10.6 -- it's crazy speedy on multicore machines, it provides the foundation for next-generation features, it lets you drag-and-drop to combine movie clips, all that good stuff.
The fact is, for sheer Swiss Army utility it can't (yet) hold a candle to the veteran QuickTime 7 Player with the QuickTime Pro upgrade. While you can use the v7 player on Snow Leopard and Lion, getting access to the Pro features still requires a $29.99 license code.
That's a shame, because it means many Mac users are missing out on most of the wonderful tricks QuickTime Player 7 can do to save you time and aggravation. Here's one example: splitting a long & large movie into segments for easier uploading or emailing. If you're running 10.7 Lion, you have access to QuickTime X's new Split Clip command; if you're running 10.6 as in the example below, you don't. (I'll tackle the step-by-step of getting Lion's version of QT X to do this in a subsequent post.)
In QuickTime X on 10.6, we've got a visual and very fast Trim tool; it shows exactly where the video will be truncated with an easy, iMovie-esque scrubber bar, and it does indeed save wicked fast once you trim your clip. Nice and handy.
While you can trim quickly this way, you can't actually split the file into two and match up frames so you don't lose anything in the middle (which you can do on 10.7). You'd have to go back, open the original movie, and try to figure out exactly where you trimmed it -- QuickTime X for 10.6 doesn't have a way to do this gracefully.
Oh, well. Good thing there's QuickTime Player 7 (in the Utilities folder, by default, on Snow Leopard) with a Pro license key. In this case, you just open the movie and use the selector tools (below the timeline) to highlight the first half of the movie that you want to save as a separate file.
Then, before you do anything else, go to File -> Save As... and save the file with a new name (ending in 'part 1' perhaps) to avoid mucking up the source file by accident. You could save the movie self-contained (all the movie data in the file; you could copy or move it to another machine or drive and it would work) or save it as a reference movie, QuickTime's version of an alias. Reference movies track your tweaks and edits to the movie without modifying the underlying data stored in the original file; this makes them extremely fast to work with and save, but you can't move them around between computers without their 'parent' files. It depends what you intend to do with the pieces; if you just want to give the movie sections separate file names to organize a long clip, reference movies will do fine.
Now you're ready to make the split clip. Under the Edit menu, choose Trim to Selection. Boom: you've got the first chunk of my movie sitting there in the window by itself. Go to the file menu and choose Save (not Save As...) and your movie is half the clip it used to be.
Here's the magic bit: head back up to the Edit menu and choose Undo Trim to Selection. Your movie is now reverted back the way it was before, first and second half, including your selection marks. Don't Save it, though! Go to the Edit menu again and choose Cut (or Delete, if you prefer). The second half of your movie -- right down to the frame where you cropped it earlier -- is now sitting in your player window.
If you figured out that the next step is "Save As..." with a new name containing 'part 2,' well done. Now you've got two separate movie files that each contain half the original movie, exactly where you want them. Remember that you'll need to save as self-contained movies if you're planning to ship those half-size files around to other people. You can repeat the cycle as many times as you need to clip your movie into the appropriate number of smaller bits.
It's worth checking out Apple's QuickTime 7 User Guide (PDF) if you're interested in more tricks you can do with QuickTime 7 Pro. Got a favorite? Let us know and we'll write it up for all to enjoy.