I see that more and more are using Macs for development.
Why is this? What advantages does the mac have?
Except for the obvious Microsoft languages, are there any that are not suitable for the mac?
And last, what is a recommended, not so pricey macbook for programming (C++, Java, Ruby, Haskell etc.)?
Any currently shipping Macintosh will be adequate for development (Mac Mini, Low end MacBook Pro) if you are a hobbyist or have small projects.
I find a 2.53Ghz MacBook Pro fine for my weekend projects, iPhone development and script hacking. For my Day job I'd have a hard time getting by with less than a top end Mac Pro with much less modest specs.
Mac is intensely used by designers with photoshop, ilustrator or after effects. Also there are more and more objective c programmers which develop iphone apps.
I believe that because the only eligible way to create iphone apps is to own a mac, developers were forced somehow to buy mac in their way to create apps.
I do .NET development regularly on my macbook pro using Parallels. The performance hit is noticeable but not enough to matter most of the time. The worst part for me is the inconsistency in moving the cursor around in applications using the keyboard (CMD + left/right, etc). I don't know if it's from me messing around with Parallels keyboard binding settings or not, but even after 6 months, I still don't know the different shortcuts as well as I do in Windows.
The Mac is also very nice for web development. It is very easy to set up a local development environment as PHP, MySQL (I think; if not there's an easy installer), and Apache come preinstalled. There is a lot of nice web development software for the Mac, such as Coda, CSSEdit, and TextMate.
As for software:
C++: Use Xcode. It's quite nice (though I hear it won't win many converts from Visual Studio. Having never used VS I can't comment on that).
Java: Use Eclipse. But then, you probably already knew that.
Ruby/Haskell: Textmate!
Also, for GHC, install MacPorts and use that to install your haskell compiler.
Lastly, don't pay Apple's prices for RAM upgrades. You can buy standard RAM from any supplier for much less.