Reddit itself provides pretty good data on subreddits in the /r/<sub-reddit>/about/traffic/. Not all subs have traffic figures available though.
Example:
https://www.reddit.com/r/PHP/about/traffic/
Redditmetrics.com has good data too but it is subscribers, not viewers.
http://redditmetrics.com/
Wow. Thanks.
related:
Word is that there is a UI makeover in the works.