Comedians wanting to advertise by having a clip of their act posted. Film and especially television show clips or references. Seeing a funny clip from Key and Peele, The Daily Show, etc makes you more likely to watch or get cable for Comedy Central. Reddit is all about obfuscation of the source, so viral marketing blends right in with everything else. This marketing is not only for products, but people (although usually in those cases the people are the products, like celebrities).

Some brands that are likely heavily involved in Reddit viral advertising:

• Lego (easy to tap into nostalgia, and Lego posts are almost always in the top weekly posts on /r/pics or /r/funny. Tesla (non-news about Tesla often tops /r/science, /r/technology and /r/news) McDonalds (videos showing how transparent they are about their ingredients, food photoshoots, marketing, etc) WalMart (no such thing as bad publicity) CostCo (numerous mentions of how well they treat and pay their employees)

And there are countless examples of flash-in-the-pan things like the Ice Soap, or the Arby's / Pepsi commercial.

But just because a post has a corporate source is not necessarily a reason to downvote it. I chuckled at the Arby's / Pepsi commercial. I like watching a short video on how McD's dresses up a burger for a photo shoot and how they photoshop it like every other advertisement. Movie trailers are interesting. An article on new technology involving batteries on a Tesla is interesting content. Users can still comment, even if their votes can't do much against however business manages to get low-content posts up voted to the top.

Despite advertising influences, that's one reason this site is still free and isn't loaded with annoying advertisements in your face or popping up all the time. People bemoan reddit's decline, but it can be a fantastic site once you get your interests nailed down and get away from the defaults you don't like.