Here is a list of websites/tools to analyze other websites' traffic and traffic sources. Few of them also provide additional details like traffic trend over a period of time, link popularity... A couple of them provide you details of keywords used in Adwords by your competitors. So, go ahead and use them for SEO and PPC and kill the competitors if you can :). If you don't beat the competition using PPC or SEO then you are giving business to your competitors. So, better know the competitor website's performance and then analyze your online marketing strategy to over shadow them. More over these tools are still free to use.
Google Trends – use commas to compare unique visitors to multiple sites.
Quantcast – Will be helpful if at all the competitors are “quantified”. Also, you need to register to get the complete report.
comScore - Is definitely not cheaper and not completely accurate either. I think they use an aggregate data, based on what is available to them.
dataopedia.com - Gathering together data from more than 50 sources, dataopedia.com is an aggregation web service that lets its users find out all the valuable facts about any website, such as traffic, online buzz, contact information, popularity in social bookmarking services…in short, all the essential facts about every website you can come across on the internet. dataopedia.com has been conceived as the one-stop-resource for finding website facts.
Let me know if I have missed any good websites here. If so, I will add them as soon as possible so and it will help all of here in the online marketing world.