Nvu - The Complete Web Authoring System for Linux
This is an open source project that extends Mozilla Composer. It doesn't really match the capabilities of FP 98 -- it's very document centric, not site-centric. It's more like the FrontPage Editor portion of FP 98. Still, that's very valuable.
It apparently runs "OK" on the Mac but you need to use the Ctrl key, not the Cmd key. It needs some more Mac work, but it's not out of beta yet. I'll try it out ...
Update: This really isn't too bad. It has some obvious bugs, but overall it works. It's not super-snappy, but I didn't run into much typelag in some quick testing. It looks like Mozilla and, yes, it's no FrontPage 98 (really the high point of FrontPage's life, later versions were inferior). The site management feature expects a real web site -- you need ftp access. There's no WebDav support. (whatever happened to WebDav, and, more importantly, why? Did WebDav die because of IP issues?)
I'll give this one some more use. I do need a viable replacement for FrontPage, though that will take a long time. (Document-centric low end web management and authoring is what's technically known as a Faughnan-market. That is, a market made up of John Faughnan and similar individuals. At last count there were five of us worldwide.)