I'd like to expand my use of fonts in my web designs. I know that there are a variety of services available like Google Web Fonts to include fonts onto my site, but I would also like to be able to use those fonts in high-fidelity design mockups made with either Fireworks or Photoshop. Is there an easy way to import web font files so they are available outside the browser?
It's a paid application, but I've found TypeDNA to be pretty useful for previewing fonts in Photoshop http://www.typedna.com/. Among it's many other useful features, it has a feature which allows you to use Google Fonts directly in Photoshop. Again, it's a $49 purchase, so this may not be what you're looking for.
Another option, which is free (at least currently), is the beta "Web Font Plugin" from Extensis http://www.extensis.com/en/webfontplugin/index.jsp. I haven't used it myself, but it looks to be pretty useful. It gives access to Google Fonts, as well as thousands of WebInk fonts (From what I can gather, WebInk is a usage-based web-font subscription service)
As for converting webfonts to be able to use them in Photoshop—I'm not sure of how to do this, but even if you could, most webfonts are not licensed to allow this.
If the typeface is open source, freeware, or public domain (all different concepts in terms of licensing, though the cost is the same (free)) then you can just download and install the font as you wish. All of the fonts Google hosts can be downloaded and installed and used in Photoshop as you see fit.
For commercial typefaces, there are typically two main options (aside from outright purchasing a license from the start):
use the online sample type-setting option. Typically a flash or server-side image or embedded web font. You can then screen shot it and use it FPO.
Ask the type foundry for a trial version. Several will allow that. Or if not a copy of the font file itself, they may be happy to set an image version of it for you.