From where do you get all the visitors to your site?
The sources from where they come are very important and regularly tracking them can give you basic idea of what kind of people visit you site/blog, helping you to further improvise on SEO. The sources of traffic can be divided into 3 groups :
1)Direct sources
2)Search engine sources
3)Referring sources
Direct sources are the visits by people who visit your site directly by entering URL ino their browser. It can include you, your friends and visitors who enter through bookmarks.
Search engine sources are the search engines which drive traffic to your site. Knowing the keywords for which you earn traffic is very important.
Referring sources are sites which contain link of your site in them. These are mainly social bookmarking sites, blog communities, social networking sites, etc.

Here are some sites that will help you determine these sources :
Google analytics
SignUp for google analytics and get detailed overview of your site and your traffic.
They give you a short code to install and you are done.

Sitemeter
SignUp for a sitemeter free counter, you get an option "By referral" in the sidebar which give you details of the sources. You also have option to see what the users clicked on your site.

Mybloglog
This is my favourite tool. Mybloglog is a blog community, if you dont have an id Signup there and add a blog/site to your profile. You get an option to know statistics of your site. This lets you know visitor sources and also what users clicked on your site.

I have listed these under traffic sources topic but they will provide you a lot more info about your visitors like :
Which keywords get you traffic from search engines.
How long they stayed on your site.
What they clicked on your site (Important specially when you use ppc ads or referral links).
pageviews within your site.
Geo-location of visitors.
Average visitors per day.

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These tools will help you a lot! trust me.