Sometimes when one Calculator application isn't enough, it's annoying that there isn't a File -> New Calculator option. The only way I could find to run two calculators is go to the Finder, then to your Applications folder. Right-click or Control-click on the Calculator application, then select Duplicate. This makes a copy of the Calculator App. You can then run Calculator Copy.app so you can have a second calculator.
[robg adds: You can use this trick with many applications, though I'd stay away from changing preference settings, and results may be less predictable for larger, more complex applications. I thought it had been discussed here previously, but all I could find was a hint on running multiple Windows Remote Desktop clients.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030512095111594