For the FOSS (Free and Open Source) side, I am investigating (in order of most promising to least):
Lightworks [10.04 beta] (Windows, Mac, Linux) Fancy, high-tech, but has only 16 AlternativeTo likes. It will truly be Open Source in about a year, IIRC.

Lightworks is an Oscar and Emmy award winning professional NLE [Non-linear editing] system for editing and mastering of movies in 2K and 4K as well as television productions in PAL, NTSC and HD. Numerous internationally renowned and Oscar award-winning feature films, including The King's Speech, Martin Scorsese's The Departed, Mission Impossible, Pulp Fiction, Braveheart and Batman were edited on Lightworks. [From Wikipedia]
But nobody cares...!
OpenShot (Linux) Featured on header on Ubuntu 11.10's Software Center
Cinelerra
Cinelerra is a highly advanced and professional video editing software, but still remains open source. Cinelerra solves three main tasks: capturing, editing and compositing. There is virtually no limit to the video resolution so whether its standard or high definition (hd) doesn't really matter in Cinelerra.
... But Lacks the polish and stylishness of Lightworks.
Kdenlive (Mac, Linux) ...Well, I have to include something by KDE, don't I?
VLMC (Windows, Mac, Linux) From the makers of VLC... I think I will like this once they come out of pre-alpha and alpha.

EDIT: I have ended up learning Lightworks. Although I haven't tried the others, I can say it has a steep learning curve.
Previously, it used to cost >$3000, I believe. But they have gone [or will go, to be pedantic] Open Source, for the most part. I don't know how it compares to other editors of the same grade, such as Avid, but nothing can compete with its new price tag. I don't think I would pay hundreds of dollars for Sony Vegas if I have Lightworks.
One feature I really liked was the curves for keyframes. Basically, you can add a curves for parameters like X position, or gain or contrast, and Lightworks will interpolate through the points. An extremely simple example would be panning from left to right or zooming in. You can add 'keyframes' for virtually any effect that has editable parameters (all of them).
I still haven't figured out the best way to insert clips or move multiple clips around/etc. If I use the 'easy method' to insert a clip in the middle of only one track, it gives me a indicator:
Current unjoined clips do not allow sync fixup.
...Which I haven't figured out.
Conclusion
I am using Lightworks, a professional NLE system. It has a steep learning curve and its own unintuitive, but potentially powerful, workflow*. But now that I am using Lightworks, I don't think I can turn back without missing the Lightworks way of doing things.
*I still haven't figured out what the 'workflow' is, but I know I will bask in the awe-inspiring beauty of it once I figure it out.