Greetings, BostonBomber, and welcome. You asked:

I have a "real" full-time job and also do some sole-proprietor-type work on the side that generates a moderate amount of income for me. I've maxxed out on the ROTH and am trying to figure out another option for retirement investing besides the 401(k) since my employer does not match.

I've done preliminary reading on the SIMPLE IRA. Does anyone have experience with this? It seems I can use this as a self-employed, sole-proprietor; however, do I need to have been making contributions all year long, or can I make a contribution at the end of the tax year?

You should read the sections on SEP, Keogh and SIMPLE plans in my Foolish Retirement Plan Primer as well as IRS Publication 560, Retirement Plans for Small Business, available at http://www.irs.ustreas.gov/forms_pubs/pubs.html. Both will give you a good overview of what's available for your self-employed money.

Be aware that a SIMPLE of Keogh contribution must be coordinated with a contribution to your employer's 401k in your other job. The contributions to these plans combined by not exceed $10,500 this year. A contribution to a SEP does not fall under this limit, so the SEP (besides being far simpler to set up and administer) may be the best bet for you in maximizing what you may put away each year between your regular job and your self-employment.

As to making the contribution as a self-employed person, you can do that up until the due date of your income tax return. You will find the deadlines explained in Pub 560. Therefore, you do not have to make those contributions based on your self-employed income until you know what that net income will be for the year.

Regards..Pixy