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This is a patch that is awaiting review, so you can't get a transparent background yet. "Always on top" and "No border" are KWin's (the window manager in KDE) options. As said, right click on window titlebar -> Advanced. You may also change the opacity of the window.
If you want the settings to be permanently, KWin lets you give the window special rules (right click on titlebar -> Advanced -> Special window settnigs...).

I'll let this idea be open to hear what other people think.

I really don't understand the objection to this idea. Plasma can already add panels that stay on top (windows go under). The only control that is missing as far as I can see is a per-panel transparency adjustment. You might want a little more transparency than the current (air) theme.

To test the usability of this idea I created a new panel and added a clock to it, made it "windows go below", resized it and moved it to an appropriate place on my screen (only restriction is that it is a screen edge)

Personally I *can* see why people would want this and I can't see why there are so many nay-sayers. Especially as the recent theme merge probably means some additional theme related parameters are going to be exposed anyway. Jeez people, relax a little. Work out how to say "yes".

Since I'm the only one who's said no(?), I guess you mean me.

Yes, I can see people using this. I can also see people wanting to configure the delay before an autohidden panel is shown, or set the exact height (in pixels) for panels. However, that doesn't mean that I approve of the ideas.

In this case I say no because I don't think this feature is useful to many users. You're welcome to prove me wrong by voting up this idea.

For now there are some workarounds to get this functionality. The one using plasmoidviewer works pretty well in my opinion, if the background just could be transparent. You also mention panels. Maybe someone could make a containment that's always on top and freely movable?

Anyway. I think it's nicer to motivate why you voted "Bad" instead of just voting, but that's just my personal opinion.

To put it simply: I think it is a corner case that would require a lot of work, a lot of changes to the user interface, will only work with compositing, will be difficult to maintain down the road, and is not of interest to many users, while good (albeit not perfect) solutions already exist. If we implement every feature users might want we will end up with a mess. We have to balance the costs and rewards of a given feature, and I think the costs here far outstrip the rewards.

Plasma is simply not designed this way, to have a single widget sit on top of all the windows while the rest are on the desktop. It would, in effect, require implementing an entire new containment for this purpose. We are not talking about a minor change here.

Well as far as I am can see it is already implemented but for an opacity control on panels. No sweat.

The plasma developers have repeatedly stated they are not going to implement opacity control for panels. Nor would it be feasible for them to do so with the svg theming plasma uses. There would have to be a new panel containment with no background.

I personally really need this kind of behaviour from certain plasmoids like the clock or the tray.
currently I am using cairo-dock and the clock and tray are floating perfectly in a quiet corner not interrupting and easy to see and click.
the only bad thing is that its not general option for any plasmoid but just for those i got with cairo-dock, and they don't follow theme style, most important the tray has background, the clock is cool and got no background.
anyway I hope this one be, I have accidently posted a similar idea and got like 12 votes in few days so I think people like it.