Be it a desktop, laptop or netbook many people I've asked this question to answer "my home one" - if that's the case for you I'd love to hear the reason why.
If you're old enough to do so please cast your mind back to your work PC circa 1995 - the vast majority of personal computers back then were desktop machines - the exception being for unusually mobile people like some sales people and some IT Professionals. Back then typical "road warriors" used laptops that were of poor performance compared to desktop machines and they cost >£1,500 each. We relied upon corporate networks for useable internet access as dial-up access was the common home alternative - broadband and 3G simply hadn't arrived.
1 - 2 - 3 back in the room! These days one can buy a perfectly good netbook for £300 and 3G (3Gb / month) mobile internet access for £15/month in the UK. If you want an uuber desirable (pretty shiney) laptop it can be yours for around £1,000. Both computing power and Internet access is no longer the luxuary they once were - I know they are still outside the reach of some people but the situation is improving rapidly.
I continue to be amazed how poor the corporate computing experience is for many people due to the provision of machines that are not fit for purpose - not due to lack of money being spent but simply due to poor configuration/management/consideration of what's required.
I've come across companies who spend over a thousand pounds per laptop per year simply to have it managed by a third party and yet they take forever to boot - run ancient software and simply don't work properly.
I'm not suggesting spending more money on management - I know it's simply not there even if one wanted to do so.
If you are an IT Professional then work with those around you to view your corporate infrastructure from the perspective of your users. Whatever your level of seniority I strongly suggest you make a few minutes to watch how end-users get on with their corporate PCs - chances are there are improvements that could be made simply by better understanding where the frustration lies.
I have seen far too many people saying "stuff it - I'll bring my own machine in" - typically netbooks - entirely bypassing much of the corporate infrastructure and therefore making it obsolete. A challenge of "letting people do their own thing" is that it becomes incredibly difficult to comply with data protection legislation plus few users backup their data effectively.
If you see people bringing their own machines in then make the time to find out why they've chosen to do so - don't start suggesting solutions - first find out if there's an effective way to improve efficiency either by improving systems or training.
Most people with good intentions disrupt systems that are broken - if corporate IT doesn't provide the tools to help users be more efficient then they'll either go elsewhere (perhaps less likely in the current financial climate) or they'll be less productive than they'd otherwise be.
Remember: Information Technology exists to make it easier for each of us to make well informed decisions