“Web-safe fonts are fonts likely to be present on a wide range of computer systems, and used by Web content authors to increase the likelihood that content will be displayed in their chosen font.” (Wikipedia) The following fonts can be considered web-safe in 2012:

font-family: Arial, Helvetica, sans-serif;
font-family: 'Arial Black', Gadget, sans-serif;
font-family: 'Bookman Old Style', serif;
font-family: 'Comic Sans MS', cursive;
font-family: Courier, monospace;
font-family: 'Courier New', Courier, monospace;
font-family: Garamond, serif;
font-family: Georgia, serif;
font-family: Impact, Charcoal, sans-serif;
font-family: 'Lucida Console', Monaco, monospace;
font-family: 'Lucida Sans Unicode', 'Lucida Grande', sans-serif;
font-family: 'MS Sans Serif', Geneva, sans-serif;
font-family: 'MS Serif', 'New York', sans-serif;
font-family: 'Palatino Linotype', 'Book Antiqua', Palatino, serif;
font-family: Symbol, sans-serif;
font-family: Tahoma, Geneva, sans-serif;
font-family: 'Times New Roman', Times, serif;
font-family: 'Trebuchet MS', Helvetica, sans-serif;
font-family: Verdana, Geneva, sans-serif;
font-family: Webdings, sans-serif;
font-family: Wingdings, 'Zapf Dingbats', sans-serif;

The following webfonts are suitable for sustainable web design, from a technical, human and user experience perspective, and meet best practices in online marketing, standard-compliance and usability:

Text: Open Sans (used by Google)
Icon: Fico, Tipogram, Pictos, Websymbols