The Computer Modern fonts, if that's what you're referring to, are free. So are many other TeX fonts. They are good for printing, but render poorly on the screen, and in some cases are only available as bitmaps or Metafont source. Metafont is scalable in theory, but the algorithm is slow so it is used to generate bitmaps in practice. It is also completely incompatible with postscript or truetype. The more common CM fonts have postscript Type1 versions available, and these are what are usually used these days, but they too are designed for printing, not for screen rendering.
In contrast, Microsoft's Verdana was originally designed specifically for screen-rendering, and the Bitstream Vera / DejaVu family likewise. Prior to Vera, Linux users had the options of bitmap X fonts, which were fine but not scalable; or the "standard" Type1 font versions from URW (Times, Helvetica, etc) which looked fuzzy (when antialiased) or blocky (when not antialiased) on screen. Vera changed that, and (under the name DejaVu) remains the standard on most linux systems. Also, linux users commonly install the MS "core fonts", but these aren't included with distros and their legal status may be unclear (Microsoft no longer offers them for free download).
From what I have seen, the Ubuntu font renders very nicely on the screen.