Here's a refresher from Kiplinger.com on SEP vs Solo 401k plans.

You have until December 31 to open the Solo 401k plan and can make your 2007 contribution up until April 15, 2008.

Some investors may want to look at a SEP IRA (Simplified Employee Pension) too, it's easy to set up and the deadline is April 15, 2008.

Contributions to a SEP are limited to 20% of your business income (which is business income minus half of your self-employment tax), up to a maximum of $45,000 for 2007. So if you earned $15,500 from self-employed freelancing, for example, you could contribute about $2,863.

The maximum contribution for a solo 401(k) also is $45,000 in 2007. However, you can contribute up to $15,500 plus 20% of your business income (defined the same way). Because the first $15,500 in a solo 401(k) is not based on a percentage of your income, you'll generally be able to contribute a lot more to a solo 401(k) than you can to a SEP, especially if you earn just a little extra money from a freelance job.