See the XNA Help documents for SpriteFont

As with most types of software, font files are licensed rather than sold. Font licenses vary from vendor to vendor, but most don't allow redistribution of the fonts, and that includes redistribution of reproductions such as bitmaps containing the rasterized character set. This is even true of many of the licenses covering fonts that Microsoft supplies with applications and Windows. Be careful, therefore, to ensure that you have the required license rights to redistribute any font you include as a bitmap containing the rasterized character set in your game!

Since the font content pipeline makes a copy of the font into a bitmap there is a chance you can fall foul of font licensing - this is discussed in this thread. 

Microsoft have released an official set of fonts that can be used with XNA framework applications.

It would seem that Larabie fonts have a very generous license.

Here's a few places with public domain fonts.
http://www.gnome.org/fonts/
http://www.aenigmafonts.com/fonts/fonts.html
http://tulrich.com/fonts/
http://www.smashingmagazine.com/category/fonts/
http://www.larabiefonts.com/ (some free some pay)
http://www.acidfonts.com/ (Check license with each font)
http://www.dafont.com/ (Check license with each font)
http://www.1001fonts.com/ (some free some cheap some more expensive)
http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&item_id=Gentium (lots of international glyphs)
http://www.webpagepublicity.com/free-fonts-s.html
http://www.fontgarden.com/

Cheap
http://www.myfonts.com/
http://www.larabiefonts.com/ (some free some pay)
http://www.1001fonts.com/ (some free some cheap some more expensive)

Licenses (check yourself, I'm reporting what I found)

http://www.fonts.com/ 
"The End User License allows you to install and use the font on up to 5 CPUs, and the intent of the license to allow you to create static graphics for view and print purposes. As long as you do not distribute the font software itself with the video game, the End User License is all you will need." from http://forums.indiegamer.com/showthread.php?t=6764
Doesn't specifically cover taking a copy of the entire font in a bitmap...

Microsoft
See http://www.microsoft.com/typography/RedistributionFAQ.mspx for most rules.
Some Microsoft fonts can be redistributed see: http://web.nickshanks.com/typography/corefonts. Links to the License document and the FAQ on these fonts. However the way *I* read the restrictions they do not allow for distribution in any other form than the original which makes them useless for XNA games. Consult your usual legal advisor.