I'm doing a little work on a website at the moment but the menu looks terrible in Linux. On Windows it is nice and fits on one line. On Linux the menu breaks to a second line and looks TERRIBLE!
The reason for this is I am using 'Trebuchet MS' as the font in my style sheet. Since Ubuntu Linux does not have that font installed, it goes with my backup of sans-serif. The best way to fix this is to install the package msttcorefonts. That will install a number of MS fonts including: Andale Mono, Arial Black, Arial, Comic Sans MS, Courier New, Georgia, Impact, Times New Roman, Trebuchet, Verdana and Webdings
And for a much more in depth read on all the fonts and what looks like what on Linux, check out this great article: Linux Font Equivalents to Popular Web Typefaces