I believe that SmartThings’ plan it to eventually have a “App Store” like setup where developers can push their apps for people to grab (and perhaps buy?). But this isn’t done yet.
Currently if you go into the IDE you can browse apps that others have shared. I wrote a little tutorial many months ago on how to do this: https://www.dropbox.com/s/jtlrls9otfwxpjy/STGetAppStart.pdf
Some of that might be slight out of date with some of the fields you need to enter when starting a smartapp but the basics are still valid.
And yeah, as @viguera said, you can also find some apps in the forum and/or github. There isn’t a great way to search right now.