Many months ago, I saw this blog post about a Monte Carlo investment simulator. I liked it right off the bat because rather than giving exact answers, it gives probabilities of outcomes instead. You can read more about Monte Carlo method at Wikipedia.

A few comments about their simulator:

• At first I was annoyed that I couldn't specify my estimated nominal return on my investments. But then I thought, well, they have probably determined the average return of a conservative portfolio over the past few decades and so its probably better that I don't have the power to change it, otherwise, I might be prone to use an overly-optimisted return
• They use 4.83% as an expected inflation rate, which "corresponds to the average inflation rate for the period of 1974 to 2004&8243; according to their instructions on their website. Isn't that a bit high? The most I have ever heard quoted is 3-4%, but almost 5%?
• I noticed that there is no way to specify a monthly cash flow into an asset. It turns out that any leftover surplus in the cash-flows tab will get transferred into assets (in the same proportion I assume).

I have been playing around with it for a while today and I plan on fine-tuning it a bit more. There are also a few other simulators that the blog post above mentions. Not sure how many of those are free though.