Hi, miro, Atanas.
There is an idea about log file in central admin.
Now it is possible to see only who logged in and logged out the site.
Would be great to have logfile that also shows user actions. For example user modified info in blog post, or addited info in editable page, or added info to catalog for some product.
With this it would be possible to get lots of information about user activity.
Hope that can be interesting for you