I'm sure that I've seen this font before; however, I totally forgot what the font's name is...

If anyone can help identify this font, I'll be grateful!

Thanks.

Droid Sans  Suggested by Heron2001 

link to that web/document?

I saw that too in Mac computers

Sorry, but it's the result of my Firefox browser's rendering of search boxes in certain websites. I tried opening those websites in Chrome and IE but the fonts both browsers used were different from the one Firefox used. I do have checked the settings in Firefox. Obviously this is not Arial, Sans-serif or some monospaced fonts. Also, though I used MacType for rendering all fonts shown on the screen to make the text more easily and comfortably to read, but again this doesn't correspond to any font MacType used.

using Firefox on Win or Mac? Anyways, most probably one of the system font is used and not website's font. Check Firefox default font properties.

I think it might be Droid Sans from Google Web fonts...