All too often search engine optimists make the mistake of focusing only on Google Rank. This leaves me wondering why would you want to discredit the second biggest search engine on the web? If you're like me you're going to want to keep track of not only Google Rank but Yahoo, and Bing Rank as well.
Another thing to remember about Yahoo is that it's not just a search engine. It's a portal site that serves many different purposes to it's users including email, news, weather, music, informative articles, blogging, digital media (flickr), web hosting and so much more. Yahoo is still a giant part of the web and not optimizing for Yahoo is cutting out a wide variety of potential customers.
With that said you should check out Creative (ctrl)'s SEO Monitor. Our tool will help you keep track of Google, Yahoo AND Bing positions, complete with historical ranking graphs that show you how you're rankings have changed over time. Reports are run daily and can be automatically emailed to you on a daily, weekly, or monthly basis. Export reports to CSV format compatible with Microsoft Excel, Crystal Reports and most other applications.