I received a great suggestion to make a instacalc bookmarklet:

Bookmarklets are special links that interact with the page you’re on. The instacalc bookmarklet makes a live, interactive window to type calculations!

Installation (Internet Explorer/FireFox/Opera/Safari):

• Right click this link: instacalc bookmarklet and Add to favorites/bookmarks. That’s it! (Click the link for a live preview).
• Put the bookmark in your toolbar for quick access.

Usage: Click the bookmark to open an instacalc window on the current page:

• Just type and go! Currently selected text is copied in by default.
• Close with the red “x” (or click the pink region). If you close the calc, click the bookmark again to continue where you left off.
• The calc should stay in the upper-right corner as you scroll the page.

Now you can get to instacalc when checking your email, posting on forums, or just surfing the web.

Bugs and Feedback: I threw this together fairly quickly — let me know if it works, if there’s bugs, or features you’d like to see (multiple rows, etc.). The bookmark always uses the latest version, so you don’t need to worry about updating it. Enjoy!