If your marginal bracket is 15% or lower, your (long term) cap gain rate is 0%.
If 25%, the LT cap gain rate is 15%.
For this reason, the second choice for most people after the 401(k) is simply an index fund intended to be held long term. People who fund the 401(k) find that their withdrawal receive no favorable rate, but are taxed as ordinary income coming out. The benefit of "deposit at 25%/ withdraw at 10%" is not the deal it should be due to this factor.
Since you mention your income - In 2013, the 15% bracket ends at a taxable $36,250. The standard deduction and exemption adds to $10,000. So, with no other credits or any deductions, it would take a gross $46,250 to land right at this 15%/25% cutoff. It's wise to use Roth for 15% money, and re-evaluate the type of account you use as your income increases. If you move on to a job that pays, say $52K, you might put about $6000 in a pretax 401(k), and at tax time, choose between Roth and Traditional IRA to stay right on that 15% line.
When discussing pre-tax 401(k)/IRA vs Roth, this mix post tax at 15%, pretax at 25% or higher is the best advice without knowing far more of one's financial details. And perfect for those just getting started, you'll retire with a pretty good mix of accounts.