The most dominant web-based reader is Google Newsgroups which took over the old DejaNews service and has about 20 years of
articles in it's archives.
This website (freeusenetnews.com)
freenews.maxbaud.net (oldest operating index of free Usenet servers)
Open Directory Free Server Listing
Some European site
freenews.maxbaud.net has a scorecard for sites listing free servers