Hi,
I am working on a user interface, and I am wondering what fonts do other software use like Photoshop, Windows, etc use in their interface. They can't be using Arial and Tahoma, right? This is a web application I am working on, and I can use @font-face.
But what about Unicode support? I would not like to put any limits on i18n.
Most software, barring major exceptions, uses the standard fonts that are determined by the platform. Windows has settings for the font used by window titles, buttons, normal text, etc. Users can customize these, and see the changes propagated to all of their applications (at least the ones that use common fonts).
Re: Unicode, the stock fonts on all of the major OSes have good Unicode support; I use Tahoma for IRC on Windows, and can view kanji, katakana, etc. and have had similar experiences with Droid Sans on Linux. I can't speak for Mac OS X, but I'm willing to bet that Unicode support is good.
Some information on fonts used in operating systems:
Windows 3.1, Windows 95, Windows 98 and Windows ME use MS Sans Serif. This is a raster font, and as such it does not play nice with sub-pixel smoothing (e.g. ClearType).
Windows 2000 and Windows XP use Tahoma. It supports a fair number of scripts, but some Asian scripts like Japanese seem to be absent. See also the full list.
Windows Vista and Windows 7 use Segoe UI. Again, here's the full list of supported scripts.
Gnome uses Bitstream Vera Sans, but this is probably distribution-dependent. A derivate font with more scripts is DejaVu Sans.
Mac OS X uses Lucida Grande.
For a web application, I'd probably stick with the font that the OS and the browser use. Something like this will probably work for most users:
font-family: "Lucida Grande", "DejaVu Sans", "Bitstream Vera Sans", "Segoe UI", "Tahoma", sans-serif;
I put the Windows fonts last, because many OS X and Linux users will have these installed, but we want their native fonts to take precedence. The reverse is less likely to happen.
Whatever you choose, use a sans-serif font. On a computer screen, these are far more legible than serif fonts.
If you must use @font-face, be sure to read the font's license before you distribute it. From the fonts mentioned above, I think only Bitstream Vera Sans and DejaVu are free to use.
In Windows Vista and 7, according to this page of docs, the default system font is segoeUI. Read the whole entry: it appears that you'll need other fonts to cover the gamut of languages...:
Segoe UI includes Latin, Greek, Cyrillic, and Arabic characters. There are new fonts, also optimized for ClearType, created for other character sets and uses. These include Meiryo for Japanese, Malgun Gothic for Korean, Microsoft JhengHei for Chinese (Traditional), Microsoft YaHei for Chinese (Simplified), Gisha for Hebrew, and Leelawadee for Thai, and the ClearType Collection fonts designed for document use.
Macs have been using Geneva for a long time (but as wikipedia says it's "not commonly available on other platforms", though the follow-on suggestion that "many find Verdana or Arial to be an acceptable substitute" would draw screams from many typography experts and fans (many of whom are in any case way bored with any Helvetica-inspired font anyway;-).
(Maybe it's time to propose a fonts-and-typography stackexchange forum? there are some programming issues connected with fonts and typography, but they're hardly core to them...).