I've recently started using Linux as something more than 'an OS to play around with'. I use the Debian distribution, which sets out it's aims to be a true Free-Software OS and set of applications. With the latest releases of Mozilla (a web browser), Evolution (an email application) and Nautilus (a flashy GUI filemanager), I belive that GNU/Linux is a viable alternative for many home users. Combine that with newbie-friendly distributions such as Mandrake and Suse and Microsoft would have some good competition, if people were educated about the options they had.
I have two Linux systems so far, both Debian, one Compaq Presario 1615 laptop, and my AMD 500 desktop machine.
Updated: My new laptop now runs Debian too, check out the Asus Laptop page for all the details.