Well, most operating systems do let you know when you've reached a critical low. However, it is up to the user to do the work of clearing out the space. I know that Windows will do what it can to assist, as does the latest Ubuntu (not sure about other Linux distros), but it can only do so much. I haven't played with anything beyond Tiger to any extent, so I don't know if those also do what they can to assist in freeing up space. Hoping to change that little problem soon, though...
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1