How can I simulate having a slow CPU? I am almost done coding my webapp-game. It runs on all modern browsers. However, I want to establish some minimum requirements for it.

Thus, I need a slow Mac. However, I don't have access to a slow Mac that has a modern browser (IE 9+, Safari 5+, Opera 11+, Chrome, Firefox 4+).

Using the tool mentioned in this question, I can simulate a slow Internet connection. I can use up plenty of memory easily. However, I have no clue how to simulate a slow CPU without resorting to underclocking.

Is there any way I can make my Mac's CPU slower temporarily without underclocking?
One solution is to use virtualization to have another instance of Mac OS X running on your computer. Many virtualization applications (such as Parallels Desktop) let you decide how much resources an instance can have (e.g. memory or cpu). Lion increased its support of virtualizing Mac OS X.

If you do decide to go with Parallels, there's a useful how to article describing how to set up an instance of another Mac OS X Lion.