According to Wikipedia Myriad Pro is bundled with Adobe Reader not with Windows.
And a quick Google search shows you can use it on the web using Typekit (with a $24.99/year subscription): http://typekit.com/fonts/myriad-pro
If your website targets the designer crowd, many of them will have the Adobe Suite installed (don't ask by what means). Kottke.org uses it without css embedding, and this is his font family rule:
font-family: MyriadPro-Regular, 'Myriad Pro Regular', MyriadPro, 'Myriad Pro', Helvetica, Arial, sans-serif;
A free font that is very, very close: Vegur
The character support isn't the best, but if it's only for headlines and it really has to be Myriad, then the extra effort of @font-face-ing it may be worth it.
You have various choices:
One day in the future you could use @font-face loading any font that you want (which license is free to distribution).
Or you could use, like already suggested payment hosting servers for font Typekit.
Vegur has a @font-face (as well as Cufon) kit at http://www.cufonfonts.com/en/font/12046/vegur. Browser support has advanced to the point that most people will be able to see the font used through @font-face even if they don't have the font installed, as long as the font is hosted on the server.
I had the same question, I settled at CartoGothic-Std after reading the following article:
The font's download link:
Open Sans would be the best alternative font for that!
Source Sans Pro is a great alternative.
It's available through Google Fonts: http://www.google.com/fonts/specimen/Source+Sans+Pro
Very similar to Myriad Pro, big family. Authored by Adobe (who also authored Myriad Pro) as its first open source type family for text.
To all the people planning to use the Google Font Directory (it rocks, I use it too sometimes). Be aware that some fonts add significant weight/loading time to your pages. In the case of the mentioned PT Sans family it is "Added page weight: 224kb compressed". Check it out here: http://www.google.com/webfonts/family?family=PT+Sans&subset=latin
If you want to use
@font-face i always look to font squirrel. you can upload fonts and they will create a set for your site with the included
One more similar/alternate of Myriad Pro is Maven Pro(google font). See Link
If Verdana is close enough, DejaVu Sans is closer. The two are compared here; note the 'I', 'Q', and 'R'.