Gnome is a reasonable desktop manager, but it doesn't seem to match Mac OSX or Windows 7. Are there any alternatives for Ubuntu that focus more on usability?
The most common other two used with Ubuntu are KDE and XFCE. Both are available as pre-packaged distributions known as Kubuntu and Xubuntu and both have live CD's available if you just want to try them out. (Of course you are free to download and install them to your existing Ubuntu install if you don't want to start from scratch).
If you are looking for something that is closer to windows 7 or OS x then KDE might be worth a look. In my opinion it has a better looking interface than Gnome. It does have it's quirks though and doesn't really have any more of a focus on "usability" than gnome does.
XFCE focuses on simplicity and productivity.
There are many others. You can find a partial list and some points of comparison on the wikipedia comparison of X-Window System desktop environments
I'm not sure that's a valid complaint. Just because something is different, doesn't mean it's less accessible or less usable.
Neither Gnome, KDE (or many other environments) are trying to be the same as Windows or Mac as they believe there are user experience and accessibility issues with them. They're both trying to be something new that works better.
Gnome plans to move even further away from both Window and Mac in future releases by bringing in an interface called Gnome Shell. You can try this out today if you wish but it's a way off from being complete.
Now, if you're looking to clone the Windows 7 or OSX interfaces on a Linux computer, that can be done with a bit of modification. Check out a site like gnome-look.
There is also the possibility of just adjusting the (gnome)desktop so it looks similar to windows or mac.
You will find them all in launchpad.net
If you want to get a Windows 7 look and feel check out this theme: http://gnome-look.org/content/show.php/Win2-7+Pack?content=113264
Really it's more of a complete make-over, so be sure to read all the instructions carefully! Even if you don't want it, it will give you insight into changes you can make to not necessarily get the look but get the feel. The AWN dock with the DockbarX applet is highly recommended as a Gnome Panel replacement.
Gnomenu can replicate the Windows 7 menu, but that's a step backwards imo. Nautilus Elementary with Gloobus Preview can give you Mac's Quick Look feature. I've never used a Mac, but I love that feature! Nautilus Actions can add items your File Manager context menu and Wine can let you install Windows apps like Utorrent. The Screenlets app will allow you to install desktop widgets.
The Global Menu applet will give you your apps' menus in the panel (I'm told this is like Mac). Window Buttons will allow you to have your close, minimize, and maximize buttons in the panel. With these two applets you can hide your maximized windows' title and menu bars giving you much more vertical space. (I'm assuming you have CCSM already?!)
I could go on, but I think that's plenty. Have fun Google-ing, and here's a few places to start:
Nevermind. I can't post more than one link because I'm told it might be spam. Whatever. Good luck.
Edit: I should note that by installing Compiz Config Settings Manager you will get, imo, functionality superior to just about everything, plus cool eyecandy. There's tiling, window placement, transparency, zoom, Expo, add and arrage virtual desktops, and more. Compiz makes Ubuntu/Gnome so easy to use that I find it literally HARD to use Windows 7 and only log into it when I have absolutely no choice (which is almost never). This is just my opinion though.
I also didn't mention Easystroke, a system wide mouse gesture program available in the repositories, which is fantastic. You can configure that to move/tile windows, open menus, go back or open new tabs in Firefox/Chrome/Opera, and so on. And don't forget about the super convenient launcher Gnome-do.
The best thing? All free and available in the repositories.
You can install either KDE or Xfce environments easily from an existing ubuntu installation or from their liveCDs. I will assume you already have a working ubuntu install and want to switch desktops without reinstalling.
For a full KDE environment plus KDE applications, install the kubuntu-desktop meta-package from the main repositories. Kubuntu is focused on being "friendly", which might match up better with your usability needs. Their website is http://kubuntu.org.
For a full Xfce environment plus lightweight applications, install the xubuntu-desktop meta-package from the 'universe' repository. Xubuntu is focused on being fast - their website is http://xubuntu.org .
These are official ubuntu packages and use most of the same underlying systems and applications as the standard Ubuntu desktop. You can install these alternative desktops easily using your package manager/software center by looking for the appropriate package listed above. Note that kubuntu-desktop in particular will bring in many applications/libraries as dependencies, since it uses the KDE equivalents of a lot of Gnome software (Koffice rather than Evolution, Amarok rather than Banshee, etc).