Is it legal for my former employer to withhold my entire final paycheck if I quit without giving 2 weeks notice?

The Fair Labor Standards Act (FLSA) requires that employers pay wages to employees for their hours worked. Under no conditions is an employer allowed to withhold payment of wages; they are violating the law if they do and there are legal penalties for doing so. The FLSA requires that employers pay employees for hours worked; however the act doesn't require that employers issue a departing employee their final paycheck immediately upon resignation (but does recommend that it be issued by the next regular payday). That having been said, state laws may require that a paycheck be paid immediatley upon resignation (it varies from state-to-state). 

The above holds true whether or not the employee gave 2 weeks notice. Absent some sort of agreement to the contrary (i.e. employment contract, union agreement or stated company policy), most employment is what is know as "at will". This means that an employer or an employee has the right to end the working relationship at any time, for any reason, with or without notice. Accordingly, the practice of giving a 2 week notice isn't a requirement, it's a professional courtesy.

If your paycheck is withheld you can file a complaint with the US Department of Labor and with your own state's labor department. Additionally, you could file suit against your former employer. To be fully certain of your rights, you should consult directly with an employment law attorney.