There really is a subreddit for everything. Examples:

• trees sucking on things
• trees sucking at things

spoofs of other subreddits

• avocados gone wild — a spoof of /r/gonewild [NSFW!]
• nongolfers — a spoof of /r/atheism

non sequitor

• animals without necks
• /r/titler [nsfw]
• more

kind of disturbing

• /r/incest

long thread, with lots of input on there being a subreddit for everything