If search engine ranking is vital to you, then you need to keep tabs on your progress. There are several free tools that allow you to do some simple checking based on keywords and single URL's. If you need more exhaustive tools, tools that provide detailed reports on all your pages, then you should consider commercial tools.

Many of these tools require a Google API Key. There are also tools for Yahoo and MSN.

Rank Pulse - very cool tool for keeping constant tabs on around around 1000 common keywords. The service is free and great for keeping tabs on your competitors.

Google Rankings - checks Google, MSN and Yahoo.

Market Leap Keyword Verification - checks to see if a page is listed in the first 3 pages of the SERPs for 7 differant search engines.

Some of the best commercial tools available, are a combination of tools in a suite:

Web CEO - awesome all in one search engine optimization tool.