One of the most important isssues in web design is the font you choose. If the font is too fancy, the viewer may not be able to read it, yet a few interesting fonts can make your site a lot nicer.

Provide alternatives!
Often, if you use a font which is not in the general selection of fonts that come with operating sytems and your viewer doesn't have it, the default font (Times New Roman in most cases) will show up. If you look at Microsoft's site and also at mirrormere.com, the Verdana font (the one you are probably seeing right now) used gives a very technical and smart feel. Some users may not have this font, so alternatives are provided so that the default font is only used if truly needed. Two similar Sans-Serif fonts to Verdana are Arial and Helvetica, so the font tag looks like this:
<font face="Verdana, Helvetica, Arial">
If Verdana is not installed, Helvetica will be used. If Helicavita is also not available, Arial will be used. Only if Verdana, Helicavita and Arial are all not installed will the default font be used.

Not too fancy please!
If your entire site is in Blackforest, for instance, it may become rather illegible for readers with impared eyesight or just those who've had a long, hard day! Stick to more easily-readable fonts such as Times New Roman, Arial, Verdana, Helvetica or other plain fonts.

Make sure your site downgrades gracefully
Don't tell users 'you must download this 3MB pack of incredibly beautiful fonts before clicking on this link', or something similar. You can tell them that they will see it better with those fonts, but you really can't expect casual viewers to download files just to see your site. At least provide alternatives.