I have had the privilege of working on the development of a set of learning objects supporting an instructional trajectory in Fractions, Periodic Functions and Linear Patterns, called CLIPS.
There has been a lot of effort to design a wrapper to complement these activities. One of the elements of this wrapper has been a simple calculator. Recently, Greg Clarke completely overhauled the CLIPS calculator to support four modes: Basic, Scientific, Graphing and Conversion. Greg was able to take advantage of the work of others who have freely shared their code in the opensource spirit. The calculator is available inside CLIPS by clicking its icon at the top right. It is also available standalone at www.oame.on.ca/clips/calculator.html. The URL can be appended with ?mode=basic or ?mode=sci or ?mode=graphing or ?mode=conv as above to jump directly to one of the versions. A drop-down list is used to switch between modes, open the calculator in another window (which makes it resizeable) and to download the .swf locally for offline use, perhaps on an interactive white board.
The CLIPS calculator is now prominently catalogued at the Mathfest wiki page of online calculators, which has lots of other interesting online tools listed.
One nice, inobtrusive feature is that when the equals sign is pressed, the answer is copied to the user's clipboard, ready to be pasted into an answer text field in CLIPS or wherever it might be required. I now have the CLIPS calculator as a link in my quick launch bar.