Do you need to run Windows software on your Mac? Maybe MS Project or something else not available natively? With the coming of Intel Macs, there are now many ways to do this. You can use Apple's solution – Boot Camp – to reserve a portion of your disk and install Windows. I usually don't recommend this solution, as it requires restarting your Mac to switch between Windows and OS X, unless you need to run very intensive software (like games). It also eats up a lot of disk.
I usually point people to VMWare Fusion or Parallels to run a Virtual Machine. There's also an open source solution, VirtualBox, which isn't quite as full-featured as Fusion or Parallels (but the price is right!). But today I want to talk about a new release of yet a different approach.
CodeWeavers solution, CrossOver, is based on the open-source program WINE (which stands for WINE Is Not an Emulator). What WINE does is take the Windows APIs (Application Programming Interfaces) – the hooks programs use to talk to Windows and get dialog boxes, windows, etc. – and convert them to OS X APIs. This can be much faster than running a Virtual Machine in some cases. However, the APIs don't always map out well. There are a lot of programs that won't run right using this approach.
So CodeWeavers has taken WINE and tailored it to make sure it works with certain programs, like Microsoft Office. Even then, it's not always able to support every feature and function, but the goal is to make programs like the ones in different versions of MS Office, Quicken, Internet Explorer (through IE 7 right now) and a few others work well. They just (yesterday) released their latest version, version 9. There's two levels of CrossOver, regular and "Pro" ($39.95 and $69.95 respectively – the differences are in the levels of support and in an extra I'll mention below). So why would you pay for it when it's based on open source? CodeWeavers has added a GUI layer to make management of your programs easier. Also, they made it easy to install on OS X (WINE was originally written for Linux, and there's a CrossOver version for Linux too). They also will provide support for a year (with the Pro version, and you can renew every year). With support comes help for getting programs running (if they're on the "supported" list). You can also vote for programs you'd like to see supported (and pledge some $ to pay for them too if it's that important to you).
OK, so why use this at all? If it's not a robust as running a virtual machine, why would I want to do it? Well, if your program will run under CrossOver, you've saved yourself the cost of a copy of Windows. Since the Windows calls are translated to OS X calls, there's no need for Windows itself. This also means little chance of a virus infection (you still need anti-virus with a virtual machine, as you're running a real copy of Windows). Malware is looking to exploit holes in the operating system, but it's expecting Windows, not the Mac OS. (I'm not implying the OS X is immune, just that any holes would not be the same as those under Windows).
It also can be faster, as I mentioned before. It's also less memory-intensive – a virtual machine needs to boot up a full copy of Windows. That's a lot of overhead just run one or two programs.
I personally don't use it much, but there are a couple of programs I use (some character generators for a role-playing game called Traveller that I sometime play) that are Windows only, and they run fine under CrossOver (other than one of them won't print properly).
The extra I told you I'd mention (see, I didn't forget!) is CrossOver Games. This comes with the Pro version or you can buy it separately for $39.95. This version of CrossOver is tailored for games such as World of Warcraft, Half-Life, Guild Wars, and others (yes, some of those have Mac versions now but didn't always).
You can download trial versions of both programs, so you can see if the software you want works OK or not before you buy. You can also check the Compatibility List to see if the program you're interested in is supported (and if so, any known problems and work-arounds).