Http Post utility

Poster seems to be a very good browser plugin to do HTTP Posts. It supports all http method calls, headers and post body formats. It's available for Firefox and Chrome for both Windows and Mac.
Link for chrome: https://chrome.google.com/webstore/detail/cdjfedloinmbppobahmonnjigpmlajcd

http://hurl.it/ is another good one if you want to try it on browser & not install a plugin.

If you want to download an app for Mac, then try out ApiKitchen. Url: http://www.macupdate.com/app/mac/39400/apikitchen. It has an online version too: http://apikitchen.com/
It's still in beta (as of 30 May 2012) but accomplishes most of the job very well.
Rest-Client is another good tool.