I recently investigated a couple of tools that allow you to model your tax return for 2009 to get a ballpark feel for what your refund or obligation will be.  One tool is offered by Intuit's TurboTax.  Check out the Get Ready for Tax Time section on their home page, as there are a couple of other modeling tools that can give you a feel for the tax implications of certain life changes.

The Tax Estimator can be found on the Tax Tips & Calculators page of H&R Block's site.  This serves a similar function to the one offered by Intuit, and in fact the results were pretty similar with both tools.

Although my results were similar, there were some tax events that I could not model with these tools.  Those events are pretty significant for my taxes, so I wouldn't consider these highly effective for any but very straightforward tax situations.  Nonetheless, I think there is some benefit in the sense that it allows people to a) learn or reinforce the impact of things such as personal exemptions and tax credits, and b) do what-if scenarios with certain tax actions to determine the relative benefit of taking specific steps to reduce the tax bill.

Of course, in both cases the companies are trying to sell software for providing tax returns.  I'm not making any recommendations here, except to say that if you choose not to use a tax preparer, software can be extremely helpful.  If you are planning to purchase tax preparation software, I recommend doing some comparison shopping through sites like Upromise, Amazon, or even the shopping section of Microsoft's Bing.