SEMRush focuses on your competition; its specialty is comprehensive keyword research. ("SEM" stands for "search engine marketing.") With a database of more than 100 million keywords, SEMRush can give you an astounding amount of information for a ton of websites [source: SEMRush]. Just type in a URL, and you'll get a list of keywords along with a stat-loaded chart that tells you how much traffic the site gets for each keyword and the site's search-engine rank. For each term you'll also see the total number of results, the amount of advertiser competition and a yearlong trend analysis. (Google's U.S. version is the default search engine, but you can change it.)
Using information from SEMRush, you can monitor your site and spy on your competition to your heart's content. Compare keyword results in organic and paid search, examine your competitors' marketing strategies, find out how much they spend on advertising, watch their daily position changes and find other potential rivals based on keyword overlap with your site. And that's just for starters. As you might expect, SEMRush doesn't come cheap: Prices range from $70 to $550 per month [source: SEMRush]. We can only imagine the data mountain you'd get for the high-end plan.