It depends on what you want to do and how structured your log files are. My web servers write json records for each log event and that makes analysis sweet and easy. I have other log files that are just text seached through the records.

Splunk is free for up to 500mb of import a day. It has a web interface and I use it to analyze web log files and other log files. I used the free one for a while and then went to paid because of my daily imports were occasionally >500MB.

I like it for my needs.