Piwik is good but it's also resource hungry,i recommended following tracker:
http://www.crawltrack.net/
They maybe dont have some advanced features like piwik but they did amazing thing to reduce unnecessary space and cpu load-instead fully writing user agent of writing for each ip like this:Mozilla/5.0 (Windows NT 6.1; WOW64; rv:11.0) Gecko/20100101 Firefox/11.0
they simply added number which represent browser group,so in mysql table it will be recorded as 1 instead full user agent info.
Plus on piwik you need to make table export if you want to see full browser statistic,on CT all data is available immediately.