If you have QuickTime 7 Pro, this is as easy as copy and paste.
- Open both videos in QuickTime
- Select and copy the contents of one video
(⌘+A,
⌘+C)
- Switch to the other video and move the playhead to the desired insertion point (e.g. the beginning or end) and paste (⌘+V)
Then you can save the combined movie.
Of course, it is likely you don't already have QuickTime 7 Pro already installed. Apple charge extra for QuickTime Pro (£20) which may not be worthwhile for you if you don't use it much. Also, if you are using Snow Leopard, that includes QuickTime X which doesn't have a Pro version available. In that case you'd need to install QuickTime 7 in addition to QuickTime X.
I imagine there has to be a way to do it with iMovie as well, but I don't have experience with that. Perhaps someone else might.
Indeed, you should use QuickTime rather than iMovie: iMovie will import (and therefore usually encode) the movie, but will also force you to export, as you have noted.
The main interest of using QuickTime is to use its editing features.
Assuming you're using QuickTime X (Snow Leopard / Lion), open your movie file, and hit ⌘T (or Edit > Trim). The timeline will morph into a yellow double-ended scrollbar so that you can trim the sequence.
Adjust it to fit the sequence you want, and validate the trimming with a click on the Trim yellow button. You can then simply save the file (⌘S), without the need for an export that will necessarily reduce quality.
Update seeing that you need to remove parts of the video.
You'll need the more advanced editing features of QuickTime 7. To install it on Snow Leopard or Lion, refer to Installing QuickTime Player 7 on Mac OS X v10.6 or Later on Apple's KB.
You'll then be able to open your video with QT7, select parts of your video with I (set input point) and O (set output point), and delete the part with a stroke of ⌫. Then, simply save the file, as said earlier.
You might need a license key for QT7, though (I don't remember if the latest versions of the OS need them or activate the Pro features for free).
Best Answer
iMovie only generates video at specific resolutions. It's intended for simple use-cases, and so has a really cut-down feature set for video editing to make it harder for novice users to make mistakes. Unfortunately this means it often doesn't do what you want.
If you're only clipping the start/end of your video, you can just use Quicktime alone to achieve this. If you need to add titles or otherwise do some editing on your video, you can: