We're having the same issue. We have a small handful of Mac users that
also would like/need this functionality. I haven't used the below link
yet but I'm going to test their demo out in the next few days.

www.telerik.com/products/sharepoint/overview.aspx

I played with the Telerik editor  briefly a couple weeks ago. There are
things I really like about it, but I was dumbfounded that it actually
inserted <font> tags instead of using CSS. The editor's toolbar is cleaner
visually than the MS toolbar but getting into the edit mode isn't as nice.
Where with MS's content area you just click in it and it goes into edit
mode, with the Telerik editor you have to do the "Modify Shared Web Part"
thing to get it to be active.

It works  and it's free so I might keep using it, but I'm keeping my eyes
open for a better alternative.

If valid, semantic markup are the goal, the two editors that I've found
that work best are XStandard and TinyMCE.

Alas, I don't think either of them are natively SharePoint compatible,
though TinyMCE, being all javascript based, shouldn't be that hard to
work into a custom Web Part.

FYI, Safari is a tough browser to please when it comes to rich
javascript apps. Its javascript support is different enough that a lot
of the vendors haven't gotten their editors to work 100% in it. If at
all possible, have your Mac users use Firefox instead...that'll open you
up to several more options.

Yeah, we've used both TinyMCE and FCKEditor for scratch built sites. All the
wysiwyg editors have their pros and cons. Maybe we'll look at building a
webpart around one of them (don't hold your breath for a release though).

yes! I completely forgot about FCKeditor. Being .net based AND opern
source, that might be the perfect candidate for integration into
Sharepoint.

The goal is an editor  that works  across browsers, and that allows me to
restrict input of native HTML.

I don't know that such an editor exists.

Across browser or across platforms? Most editors I've seen Tend to aim
for IE and Firefox support, with Opera and Safari support being 'nice if
we get the time'.

Outside of SharePoint, I've found it easier to get Mac Folks to just use
Firefox for their web admin tools rather than trying to find apps that
work specifically for Safari.

In the end, both Xstandard and TinyMCE will work cross platform in
Firefox and both allow you to restrict the particular HTML that is added
(with Xstandard giving you the most control over that). Maybe contact
the vendors to see if they have plans for Sharepoint versions (if they
don't, they should!)

That is the one I tried and it didn't quite work for us.

Our problem with it is it breaks WYSIWYG for both PC and Mac

TinyMCE integrates the rich  text into any textarea box. I added it to my
master page and it showed up. An asp control to print out the right
textarea page would work with TinyMCE I think but there isn't much
control on the wrapping of the toolbar within editable regions.

FWIW Xstandard just contacted me to tell me they do not supply CMS
integration code.