Online paid plans are for demanding team work purposes as they offer full packages, almost no limitations. Personally, I’d recommend UX Pin as in price to value ratio I find it the most efficient tool. When it comes to free offers on the top of my list is Wireframe.cc because of simplicity. Also, it’s intuitive to use even for a casual user. Don’t forget this list was made for the companies’ online collaboration purposes. Freelancers should have a look at those tools from a bit different angle. I’m sure they’ll find an offer that suits them better the above ones. If you’re still having second thoughts - use a trial (some of them last 30 days!). It’s the best way to learn which tool works for you best. Also, ask about discounts even if there’s no such info on a website. Sellers always take care of their customers :)