Some things to consider when looking at analyzers:
- Where is your NetFlow data coming from? If you've already got routers and switches that export NetFlow, you're probably in good shape, but if not, there are a number of free flow exporters available as software.
- Are you looking to buy a ready-to-deploy box, or a software solution to run on hardware that you provide yourself?
- How long of a data history do you need? Are you looking for a full-fidelity store, or are you OK with aggregation?
The company I work for produces a NetFlow analyzer called FlowTraq. For obvious reasons, I'm a fan :)
Other commercial offerings include SolarWinds, Arbor Networks, and Lancope. I believe Cisco has their own offerings as well. nTop and SiLK are two good open source tools; even if you wind up going with a commercial tool, I recommend trying them out just to get familiar with the terminology, and figure out what features you need in a NetFlow tool.