This piece of nifty software from Wondershare isn't free. Still, it more than makes up for its $40 (as of this writing) price tag through a streamlined interface and speed relative to the other tools listed here. Forget about just simple modifications, the software lets you build entirely new edits into your resulting MKV.
Even importing proved very easy. All you have to do is copy files right into a folder that the program always watches for new content. Then you can drag-or-drop the imported video or audio onto the main timeline, which represents the final video cut as envisioned by you. There's support for over fifty different video effects, forty-eight transitions, and text overlays.
That's the good news, but there's some bad news, too. First, while laying out video clips is fast, we noticed some lag when previewing the edit. A more notable issue: we couldn't find a way to save our edits, so it looks like you'll have to recreate your edits every time you open the program. Lastly, if you create MKVs with different video quality or resolution than your source content, you'll end up with lower quality results.