As a long time PC user (Windows and Debian/Ubuntu) coming to the Mac was not without some fear, now that I have done it, for the mobile and web developers out there heres my top pieces of software thus far
1) Textmate
http://macromates.com/

This Text editor in itself is almost reason to switch to Mac
2) Timemachine
http://www.apple.com/macosx/features/timemachine.html

Built into Leopard, having what I assume is an Rsync based backup system built into the operating system is something all operating systems should do, especially with how cheap storage is these days and how imporant the data that many people are storing on relatively fragile laptops!
3) Coda
http://www.panic.com/coda/
text editor + file transfer + svn + css + terminal
This is a true joy to use, if anyone knows anything similar for WIndows please let me know, its great to have all the tools that you use when doing web development in one place
4) Skype
Same as the PC version really, and as a heavy Skype user, I was very pleased that everything worked flawlessly and out of the box
5) Adium
www.adiumx.com
Supports the rest of the instant messaging clients that you are likely to need in one handy application
6) Preview
Again part of Leopard will open most pdf and image formats it seems, and seems to be a lot smoother scrolling through large PDFs and theres a handy preview of each page on the right
7) Parallels
http://www.parallels.com/
For those apps that havent made it to OSX or you cannot find this really does everything it claims to which I was very suprised at!
8 ) Xcode
True pleasure to use the inbuilt documentation makes the progression to Objective C a very painless one



great list. I like vmware fusion over parallels.
I second the vmware fusion over parallels.
I’d also suggest:
Skype
GitX
XMind
The Unarchiver
I don’t own a mac, but I think aMSN @ http://www.amsn-project.net/ beats Adium
CSSEdit is great for writing out your css layouts.
Yummy FTP or CyberDuck for FTP.
iTerm for terminal.
Quicksilver for quick launch application utility.
Good list overall.
eclipse is the best IDE for Mac too!