The browser-based ones have improved quite a bit recently. I use Advanced REST Client whenever I'm on a Windows machine - it is quite good.
There are a few features that my tool has that make it worth a few bucks for some people, I think, most notably being able to save requests and share them with other members of your team. I've heard from a few folks who have them checked into source control as part of manual testing scripts (there are some interesting opportunities to automate this kind of testing, too, but I haven't had time to focus on it).
And I'm releasing a feature soon that will let you paste in cURL commands and turn them into requests, which is handy when you're working with a webservice that has examples of requests given in the form of cURL commands (which is a lot of them).
Even if you're competing against free apps that match and even exceed what you can offer in some areas (which is frankly going to be more and more common), you can still find little features that might be fairly simple but add a ton of value for a certain subset of people.