The IOSA Radiocarbon Calibration Library (IOSACal) is an open source calibration software. IOSACal is meant to be used from the command line and installation, while straightforward for GNU/Linux users, is certainly not as easy as common desktop apps. To overcome this inconvenience, I dedicated some efforts to develop a version that is immediately usable.
The IOSACal web app is online at https://iosacal.herokuapp.com/.
This is a demo service, so it runs on the free tier of the commercial Heroku platform and it may take some time to load the first time you visit the website. It is updated to run with the latest version of the software (at this time, IOSACal 0.4.1, released in May).
Since it may be interesting to try the app even if you don’t have a radiocarbon date at hand, at the click of a button you can randomly pick one from the open data Mediterranean Radiocarbon dates database, and the form will be filled for you.
Unfortunately, at this time it is not possible to calibrate or plot multiple dates in the web interface (but the command-line program is perfectly capable of that).