We know that Alexa data should be taken with a grain of salt, however here is a fun little application called Alexaholic that you could use to keep an eye on your competitors success, relative to your own, or your clients.

“Webmasters, SEO/SEM specialists, and domain owners can compare and measure website statistics for up to five domains at once, switch traffic chart types and ranges without page loads, and generate report pages that are easily bookmarked and shared”