To show as many reddits at a time as possible, there is the cloud view. The more subscribers, the bigger the font; the more recent the last submission, the darker the font. You can also view the reddits as a list sorted by subscriber number. When you click a on a subreddit name a description appears, and clicking on the (+) gets the current top five submissions from reddit. About half of all reddits have no submissions, these are not listed here. About half have a couple of subscribers or less. To see how many reddits have very few subscribers, have a look at this graph. Most data is not pulled live from reddit, but saved for some time, so the numbers can be a bit off.
You can tag subreddits, and you can vote the tags on a subreddit up or down. Tags are green and in square brackets to distinguish them from subreddits. Tags with a positive score appear in /tags. This is a work in progress. Based on the tags, something like karmanaut's reddit map, but interactive, would be nice.
Some reddits have custom logos in their headers. This feature lets you browse the biggest couple of thousand reddits by their logos.
Reddit's own search doesn't work very well because it only looks in titles, and not in the comments. Google indexes more of reddit but searching there involves a lot of typing, this is what you can do here, quicker. Basically, searching for "cats" in both the "pics" and "WTF" subreddits is equivalent to this in Google: "cats site:reddit.com/r/pics/comments OR site:reddit.com/r/WTF/comments" Additionally, the results are filtered to remove duplicates and other unwanted stuff, links to mobile pages are changed to point to the normal pages, and the addresses shown always include the subreddit. For your convenience, there is a search plugin.
This lets you monitor all new comments, submission titles and descriptions posted to reddit for the mention of a word or phrase. It allows you to find out when people talk about you, a reddit you created, or a topic that interests you. Monitoring starts when you enter a term, and stops when nobody requested that information for some time. If you are logged in to metareddit, for every term you enter, you have the option add it as a summary on your (private) userpage; just click your name in the top right corner.
To get an idea in which reddits a redditor is active, their recent comments and submissions are scanned.
The list of reddits you're subscribed to is private, and there is no convenient way to share it. Until now, that is. Enter /format: basically, you provide the output of http://www.reddit.com/reddits/mine/subscriber.json, and after highlighting or removing reddits you get nicely formatted markdown code to post on reddit. No tinfoil hats are harmed running this script: your data never leaves your browser. Chrome is the easiest browser for this job: just triple-click the top box, and drag-and-drop into the lower box. Repeat if you have more than 100 reddits. With other browsers, you have to open the subscriber.json file you get when you click.