I know you can target browsers with conditional comments, as explained here in MSDN.
But I just saw one that looked like this <!--[if gte mso 9]>
What does this check for and what (other than browsers) can you check for with conditional comments?
EDIT
Is there any documentation on the conditional comments. What options and what version numbers you can use?
mso = Microsoft Outlook
9 = 2007
This German version of Wikipedia lists the various targets (IE, Microsoft Office, and VML).
http://de.wikipedia.org/wiki/Conditional_Comments (You will need to Google translate)
Also, this post suggests that MS Publisher can also be targeted
http://www.adras.com/can-t-view-page-in-browser.t32980-10-2.html