TL;DR: I just added a new feature to help discover new subreddits. You don't need to log in or anything, so just go try it out now, or keep reading if you're curious about just how it works.
The data used here is the same as for the subreddit suggestions feature, and in fact it's almost the same widget, with a slightly different interface.
Where's the data from?
The suggestions come from dataset that I build out of the usage data from the analysis page. For every session that searches for any given pair of subreddits, I assign a weight of 1. So, if lots and lots of people care about the best time to post on both /r/geek and /r/technology, those subreddits will come up as suggestions for each other. In other words, I build a weighted graph out of these searches.
In other other words, yes, I've been watching you.
(Well, all the session data is anonymized, and I'm not looking that closely anyhow).
Cleaning up the data
These suggestions are almost usable by themselves, but popular subreddits get disproportional representation. For example, if lots of people checked out
/r/technology but most of those didn't check
/r/geek, then perhaps they're not that good a suggestion for each other, and it's just that
/r/technology is a big and popular sub that lots of people search for (and some of those people happen to look up /r/geek, too). So, I also normalize the weights to account for subreddit popularity.
(In precise terms, for each pair of subreddits, I use the ratio of
(# of times people searched for both subs) /
(# of times people searched for either sub) to normalize the weight and make sure that genuinely strong connections come before popular ones.)
You don't need to understand the above, although learning is important. What you need to know is that the Discover Subreddits Tool is totally sweet and lots of fun, too.