[dba-Tech] UI Designer Tools

In a fit of creativity (must be planet-alignment or something), I've
suddenly got three ideas for apps. I'm looking for a tool that will allow
me to sketch these ideas; the current tool is a pen and a notepad. What I
do not want is any commitment to any platform. I know Access pretty well,
for example, but I don't want to embed the design ideas there, I want them
separate from any given platform and/or OS. All I want to think about is
the UI, and the progression from this screen to that and then the next, and
so on.

It's been years since I fired up Visio, and I vaguely recall that it had
some of the things I'm looking for, but that choice is already a commitment
to Windows, which is precisely the sort of thing I want to avoid.

I have a genuine reason for needing this. I think that I have stumbled upon
a new idea for a UI and I need to be able to sketch it out in a RAP (rapid
app prototyping) tool, somewhat more advanced than my current pen+paper
device. It is not important that the tool deliver forms, etc.; it will be
quite enough if it lets me sketch said forms and their progressions,
without commitment to any language or platform. Those decisions can come
much later in the game.

Any suggestions for such a tool?

TIA,
Arthur