My philosophy (opinionated as it is... maybe I drank too much 'cool aid'?); I think Apple does a pretty good job of inspecting apps at its Store, developers have a very high stake in making sure their files are 'pure'. c|net, softpedia, macupdates?
It's just very tempting for some download sites to accept less than 'pure' files when some one wants to pay them to put something up, and be vary wary of sites that offer apps that are out-of-date, especially 'brand-name' software at ridiculous markdowns. Of course, there are actually developers who may be crooks!
It's a jungle out there!
"Genio/Genieo" is included in some 'installers' without the users knowledge. It's not technically malware but it is certainly annoying and uses System locations/resources dangerously. It's all about money; the developer who allows it to be in their installer makes money from the 'extra' software, the developers of Genio make money from the sites they point to, the sites make money from the extra hits they get...