The Google Play Store has quite stealthily started showing "Recommended for You" sections today. Tapping into the recommendation area (found inside Apps, Music, Magazines, etc.) you'll see a list of apps based on a factors described within each entry. For example if you're recommended a camera app, it may say that the app is popular with users of another camera app which you've purchased previously. Other recommendations come from the nondescript "popular with similar users", "+1'd by your friends" and "popular in your area". The exact metrics for recommendations aren't known, but if we had to guess we'd say Google has a pretty sophisticated algorithm working on this.
Thankfully, you can help Google define what you'd like to see in your recommended list by hitting the hide button on any app that you'd prefer not to see from that point out. It's yet to be seen whether or not that will permanently hide the recommendation forever or if it'll come back eventually, but it's nice to know what you can influence the selections a little bit. What it all comes down to is increasing app discovery, which is a good thing. There are a ton of really great apps out there in the Play Store that just don't get enough attention.