Every visit to a your site are recorded in an "access log file" what is a source of very useful information: when, from where and how long anywho visited your site.

Where do people find link to your site?
1.Your Banner Ads (banner exchanges, button exchanges). You can evaluate what your banners are most effective and what banner exchange is most appropriate for you.

2.Listings in the big directories (Yahoo, Open Project, About.com, LookSmart, Go.com) became more important because site has targeted visitors and a most of search engines use directories as part of their indexes.

3.Text Links. Links Popularity is one of the ranking tools for many Search Engines. Classified ads, newsgroups, even reciprocal links are improving your visibility. The more text links to your site is the better.

4.Search Engines. This is a big and a separate talk. If your site isn't so famous as Yahoo, CNN or Go.com then up to 85% of visitors will find you via search engines. Conclusion: your site has to be not only visitor-friendly but robot-friendly too (Metatags, exact Title and Keywords,...).

5.Other promotion tools - Forums, Ezines, E-mail ads, BBS and so on - may be important for you may be not. It depends on product or services what you sell.

You and only you can compare effectivity of the use of any promotion means and lead your ad company.
If your traffic more than 10 visitors/day you need software, of course.

A most of web hosts supply their customer with WUSAGE or any Log Analysis Software but give results weekly. Counters and Trackers reviewed in counters.

Analog is popular free software for logfile analysis. It is able to make reports for every 5 minutes. It's available for Windows and Unix platforms.

Coast Web Master combines rich log file analysis capabilities with site mapping, verification amd more. Evaluation copy is free.
Enterprise Reporter is a log file analysis tool able to work with any server and to make accurate Advertisement report about Ad Views, Ad Clicks,...

Lists and reviews of Log File Analysis software you can find in Access Log Analyzers, Log Analysis and Statistics Software, DaveCentral Web Page Statistics.