A friend of mine just took the Mac plunge so I've been sending him links about useful Mac software I use most frequently. I thought it might be nice to collect all those recommendations here.
ClamXav is a high performance virus scanner for Macs. With the Mac's steadily increasing market share, it's becoming a more attractive target for malware authors.
TNEF's Enough is a handy little utility which unpacks the strange attachments which some Windows mail programs use (usually called winmail.dat).
MacJanitor allows you to manually run the 3 cron jobs (daily, weekly, and monthly) used to do periodic maintenance on Macs. This can be important on laptops which typically don't get left on all the time or desktop systems which may be asleep when the scheduled time passes.
Flip4Mac is a free QuickTime component that allows you to play WMV files.
JView is the fastest image viewer I've found.
MonoLingual is a utility to remove unnecessary language files on Mac OSX. This can save a fair amount of disk space.
NetNewsWire is the best RSS reader I've found for the Mac.
Transmission is a good open source BitTorrent client.
The VLC media player can play most types of video files.
NeoOffice is the Mac port of OpenOffice, an excellent and free alternative to Microsoft Office.
Here's an article on how to run Windows or Linux on your Mac in a virtual machine using some free software called VirtualBox. VirtualBox isn't the only virtualization software available for the Mac. At home I use Parallels which was the first virtualization software available for the Intel based Macs. At work I use VmWare Fusion.