These are some suggested fonts for use with Sword tools. Any of these work with either Linux or Windows tools, and probably Macs as well, though certain fonts work better with different tools. They're TrueType fonts, which means that usually just copying *.ttf to the right place makes them available, and that's necessary only if you don't have some sort of package manager or font installer to do it for you.
"Linux Libertine" is a very pretty font which provides quality character sets for all of Latin, Hebrew, and Greek, which makes it especially useful for commentaries which use all 3 at once. While it is designed for Linux, it works quite well under Windows as a display font for Sword tools. It can be found in e.g. Fedora repositories as linux-libertine-fonts-2.4.9-1.fc7.noarch.rpm, or you can get it directly from SourceForge: . MS Arial is a useful font - if not the prettiest, it is one of the few combining decent Arabic and Farsi glyph shaping with more or less complete Latinate and Greek alphabets.
There is quite a selection of other fonts available from SIL. See  and look around.
"Gentium" is a good font for Greek. Look in repositories for gentium-fonts-1.02-5.fc7.noarch.rpm or go instead to  to get the *.zip.
"Ezra SIL" is SIL's best font for Hebrew and works very well in most Sword front-ends. 
For BibleTime, the best Hebrew fonts are the Culmus fonts, particularly the Frank Ruehl CLM or Drugulin CLM. Many Linux distributions have a Culmus fonts package (Mandriva calls it fonts-type1-hebrew). If your distribution doesn't have this package, you may check out the Culmus Project site  and follow this link  to a truetype download.
For Vietnamese, fonts like Arial and Linux Libertine work well. However, the UVN fonts are excellent options that are designed for Vietnamese and can be downloaded for free. They can be used for English or other languages that use a Latin alphabet. UVN Saigon looks particularly good with many Sword front-ends. You can find these fonts at the TTi website .
Apart from MS Arial, which is mentioned above as a good all round font, the prettiest free font is probably Nazli, available from Farsiweb and part of many Linux repositories.