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.
GNU FreeFont, consisting of serif, sans serif, and monospaced typefaces in regular, bold, italic, and bold italic fonts, is an attractive set of open source fonts covering a broad range of scripts in the first two Unicode planes. Many of the glyphs incorporated into FreeFont were designed by professional type foundries.
- A derivative of the above Free Serif typeface, FreeIdgSerif, was designed for Indo-Eurpoeanists, but has wide character coverage of use to Bible users.
Linux Libertine is a very pretty typeface that provides quality character sets for all of Latin, Hebrew, and Greek, which makes it especially useful for commentaries that use all three. While it is designed for Linux, it works quite well under Windows, as a display typeface for SWORD tools.
Arial Unicode MS 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. Visit SIL and look around, but the Charis SIL and Doulos SIL typefaces, in particular were designed with fairly broad coverage in mind.
Other nice fonts with wide character coverage require purchase or registration. These are not endorsed by CrossWire, though some of us certainly use them.
TITUS Cyberbit Unicode (requires registration to download, otherwise free) -- 36161 codepoints, designed for Indo-Europeanists & medievalists.
Code2000 (free to try, $5 to register if you can afford it) -- aims at complete coverage of the Unicode Basic Multilingual Plane (BMP) aka Plane 0. Also available from this page are the freeware fonts Code2001 and Code2002, which are Plane 1 and Plane 2 fonts, respectively.
ALPHABETUM (limited trial font which is missing glyphs, €15 to purchase full font) -- a good font for a number of ancient languages such as Gothic, Old Church Slavonic (incl. Glagolitic), Ugaritic, & Phoenician
"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.
SIL Padauk is an excellent font . You may need SIL's graphite engine to properly see this font.