Don't Touch a Thing
If your employer's plan allows it, you may be able to leave your account where it is, though you won't be able to continue making contributions to it. However, you'll be stuck dealing with whatever limited service is offered to ex-employees.
Move It to Your New Employer
If you're still working, a new employer might allow you to simply transfer your old account balance to the plan they offer their employees. Not all employers allow you to do it, but it's worth asking about.
Raid the Piggy Bank (Cash It Out)
You have the right to cash out your old employer plan and take a check. This is probably your worst move because the financial repercussions are serious (and permanent).
The Not-So-Simple Indirect IRA Rollover
You have the option to take a check from your old plan and roll it over into an IRA within 60 days. Some people like the idea of having a free 60-day loan, but indirect rollovers are fraught with potential (and expensive) mistakes.
Direct Rollover to an IRA
Directly rolling over your account assets to an IRA is a seamless process that avoids all the limits placed on rollovers by the IRS. It gives you all the benefits of your own, personalized IRA without the pitfalls of an indirect rollover.