Are there any Vi clones that are still in wide use today (besides Viper mode in Emacs ;)), other than Vim? If so, what are the pros/cons to using it instead of Vim?
It's not strictly a clone, but real live no-kidding vi is available open-source now. Also lots of sources and links to many versions on the vi-lovers page.
I used the vi-plugin in Eclipse a while and it is quite good. But since my colleagues don't have a clue about vi they ran in to problems when they were trying to edit on my workstation.
From the Vi Lovers Home Page - http://thomer.com/vi/vi.html#versions
I don't know what your requirements or expectations are, but Wikipedia has a great comparison page here that should help you figure out which version is best for you.
I use Vim for all my editing except when I use Visual Studio. Fortunately there's the ViEmu plug-in for Visual Studio. Unfortunately it is not free nor a complete Vim implementation, but it's not expensive either and it close enough for my needs.
Visual Slickedit ($$$) has a very nice VI emulation mode. Vslick has other features that make it quite nice as a programmers' editor (e.g., dynamic tag generation instead of having to re-run ctags).
I used to use elvis
to satisfy my enhanced-vi needs, but can't really offer a comparison (other than that both are very good) because I switched to vim
a couple years back for the sake of using something that was easier to find support for (if only because it's what everyone else uses).
I often use nvi when I want a stripped down version of vi.
Not any real pros of using it instead of vim. More lightweight and doesn't load all the plugins I have with vim.
Vim still has lot more features (tabs, plugins, better multi-buffer support...).