I've used CamBam a good bit. You get 30 completely unlimited uses before you have to buy a license, and even then it's not that expensive considering what it can do.
Although I am looking at moving to BobCad, however it's much more expensive, and only really worth it if you need the extra features, but I'd say get experience using something free/cheaper first.
I personally have a multi-PC HSMAdvisor license, although I think they've since dropped that option. They also have a free web-based calculator called FSWizard, which covers all the basics.