MultiDesk was one of the first tools I reviewed here at CFS, because I use it all the time. I used it up until November 2006, anyway, when I found VirtuaWin, thanks to a recent discussion of similar software over at Slashdot. VirtuaWin had 2 things that I'd been looking for in virtual desktop software: source code, and active development (MultiDesk hasn't been updated, it seems, since 1999). So I tried it, and I liked it, and it has replaced MultiDesk on my computer. (If you're unsure what virtual (or multiple) desktop software is, check out my MultiDesk review.)
Two of my favorite features of VirtuaWin (as compared to MultiDesk) are the ability to use your mouse to switch to different desktops (or to drag windows to another desktop), and the ability to use custom icons in the system tray. If you enable mouse support in VirtuaWin, moving your mouse to the left or right edge of your screen will flip you to the next desktop in that direction. You can also configure your virtual desktop layout to act like a grid (of desktops) instead of a line, allowing you to move your mouse to the top or bottom of your screen to flip to the desktop that is "above" or "below" the current one. If you click and drag a window to the edge, it also pops onto the new desktop. While I personally use the hotkeys primarily, I like this mouse-switching feature (but it takes some getting used to).
The standard installation of VirtuaWin shows a single icon in the system tray to tell you which desktop you're currently viewing. On the VirtuaWin Web site, you can download a variety of icon sets to customize how VirtuaWin shows you this. Some use numbers (desktop 1 through 9), while others use geometry (to show, for example, that you're viewing the top-right desktop), and still others use symbols or pictures (the members of Kiss, or Star Trek characters). There are quite a variety of icon sets to play with on the Web site. I, of course, decided to try and create my own; the results are shown in the 2nd screenshot to the right (the blue and purple spheres with numbers in them).
To get the icon configuration you see in that screenshot, I had to install one of the many modules available on VirtuaWin's site. The module I used is called "Cool Desktop Switcher", and it changes VirtuaWin to show an icon for each desktop in the system tray (instead of just a single icon). It also adds the ability to send an application's window to a specific desktop by highlighting the window and control-clicking on the desktop icon you want to send it to. MultiDesk has this feature, and I really missed it when I first made the switch to VirtuaWin. There are other modules that allow the background image to change for each virtual desktop, or to play a noise when you switch desktops, or to give each desktop a name that will pop up when you switch. (Note that the background switching module SwitchDesk doesn't seem to run on Windows XP, which is a bummer.) If you dabble in programming, you could even create your own modules to change the behavior of VirtuaWin (I haven't gone that far myself yet.)
VirtuaWin has a lot of configuration options, and fortunately the help included in the program does a decent job of explaining what they do. VirtuaWin has been as stable for me as MultiDesk was (no crashes or problems of any kind), and I was able to get it working nearly identically to how I was previously using MultiDesk (once I added the Cool Desktop Switcher module). One thing I have noticed is that sometimes when you get a pop-up alert from an application on another desktop and you click on it to close it, that whole application will move to the current desktop and I'll have to send it back. But other than that, I'm sold on VirtuaWin and haven't had any reason to switch back. And since there seems to be an active community of people working on and with VirtuaWin, I hope we can only expect even better things from it in the future.
By the way, if you want to download my icons to use for VirtuaWin, you can get them here. This .zip file includes the blue and purple icons you can see in the screenshot, plus red, green, and yellow icons, so you can mix and match them. In the Cool Desktop Switcher configuration screen, you have to manually set up the icon to use for each desktop in each of the "active" and "inactive" states, which can take a little while to set up.