A friend of mine asked me today "Do you know a software / service to (centrally) maintain bookmarks across browsers and machines?". No, I didn't, but I went straight to Sourceforge and did a search which turned up SiteBar (the marchitecture name is ":: SiteBar :: The Bookmark Server for Personal and Team Use", but you already know that from the title). Of course I took them up on the "test on public servers" offer, and here are screenshots for both Firefox and Internet Explorer:
SiteBar for Firefox is an extension, the IE screenshot only shows the "quick & dirty" solution, not the fully integrated one (hey, I don't use IE that much any more, so why bother?). So far, SiteBar looks very promising. Oh, and btw unless you already guessed it from the marchitecture name: you can set up your own SiteBar server in your company!