That's a pretty broad question, and I don't know if many people will answer you seeing as it's integral to their business strategy: I mean would you give away your trade secrets in making your business successful?
That said, the plugin I would recommend first and foremost is one developed especially for JobRoller by one of the community members here (it's called fxtender by Sebet). It fixes/adds a lot of things that JR developers overlooked in the first instance AND adds a ton of functionality (some of which haven't eevn been mentioned yet). My particular favourite is the addition of multiple gateways.
Beyond that, there's another chap on here (Mr.Hat) that speak a lot about marketing of your JR website and has some really good advice and how-to guides - particularly email advertising.
Personally I tend to stay away from plugins as much as possible, just because it really has negative impact on the loadspeed of my website. On my jobsite that I'm running, I've probably got 3-4 plugins installed.
As an idea to figure what people have got on their website, my suggestion would be to find the high-performing JR website on here (usually the guys that are most active in the forum) and look at their code. In the head section of the document you'll find the tell-tale signs of what's installed by look for links to css/
js files.
Hope it helps.
