I agree with cs409, google analytics is very good BUT it has it's drawbacks, firstly it's not accepting any new signups and also it only updates data at selected intervals. I personally use google analytics in conjunction with statcounter.com, both are invisible to users. Google Analytics is especially good if you have an adwords/sense campaign and is also very good for tracking search behavious but for unique visitors, browser stats etc. i would recommend statcounter.