You can see the traditional cross platform system fonts like Arial and these , but in 2011 with Leopard/Lion and Vista/7, surely there are a load more fonts we can use reliably?
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The easy answer is to keep to OpenType (.otf) fonts. These are all inherently cross-platform (Mac OS, Windows, Linux). They can have Postscript or TrueType outlines internally. The only bit of (annoying) confusion comes in where some OpenType fonts with TrueType outlines are given a .ttf extension. You can determine if a .ttf is OpenType by looking at the font properties.
They are also feature-rich by comparison with Type 1 or TrueType fonts, which are limited as to character set. In order to create good typography with earlier font formats, you had to use multiple font files (standard character font, an alternate glyphs font, a small caps font, a ligature font, etc.), where OpenType fonts can have all of these, plus multiple language support, built in.
I work almost exclusively with OpenType fonts, partly to avoid compatibility issues, partly because it's way faster to set text when you don't have to switch fonts just to add small caps, a special glyph or a swash capital.
OpenType fonts are at their best when used with fully OpenType-aware applications such as Adobe Illustrator, Adobe InDesign or Quark Xpress. Photoshop has some OpenType support, but still lacks a glyphs panel.
I really don't think the state of cross platform fonts has changed much at all over the last decade. Most of the fonts that ship with the platforms are just about the same as they were in 1997 (yes there are some exceptions). However - they haven't changed enough that you could reliably use a font other than Arial, Helvetica, Georgia, Verdana, Trebuchet, etc... and expect it to be available on all devices.
And I doubt this trend will change any time soon:
Windows 7 has introduced some rather nice typefaces. I'd definitely look into using some of them in your font-family property list.
OSX hasn't added many typefaces over the years, so you're still stuck with the usual.
As other's have stated, @font-face CSS embedding has improved quite a bit, so you may have some options there too.