The well-known Patch Tuesday ritual almost exclusively targets Microsoft's most popular products -- Internet Explorer, Windows, Office and .NET Framework. What's not so well-known is the fact that Microsoft also leaves a portion of its known vulnerabilities unfixed.
Most of these unfixed flaws have been known about for years and Microsoft simply ignored them. The fact that they're out in the open (that is, on the Secunia Report and the like) increases their risk.
Here's a run-down of the most popular programs with unpatched flaws (both minor and major) as well as a quick evaluation of when this might affect either you or any one of your users.