There are several academic viruses for non-Windows systems out there, plus maybe a few actual ones. The rest are all just exploits and root-kits that typically don't fall into the "virus" category. Non-Windows-based worms are almost exclusive to Apache (and within that category, heavily favoring PHP exploits).
This isn't because it's easier to make Windows virii. Windows still accounts for the overwhelming majority of non-tech-savvy users' systems, and that's what malware writers want to target. That said, there is a growing volume of browser-based malware these days, and the popularity of Flash and Javascript over ActiveX (thanks in part to Firefox) means that most of it will work on any operating system. Since Windows is pretty much the only system that runs things permissively, damage is limited (but still quite real) on non-Windows systems. (Plus, anything trying to lodge itself in Windows paths like C:\Windows or the Windows registry or via a binary or Windows-dependent script will fail right off the bat.) Aside from Javascript issues, I've never heard of a non-Windows piece of malware that spread through email. I don't know of any email clients that support Javascript any more, and any sane webmail server will defang in that regard as well. That largely limits non-Windows malware to "click here" items, for which we have the URI blocklists. In summary: don't run things as root, keep up with your distro's security updates, don't serve CGI outside of localhost on your non-server, and be careful where you point your web browser. For Linux, I also recommend fail2ban, http://www.fail2ban.org/
