That's an important question which each individual will approach differently. The first consideration is whether or not certification in general is the right solution. Generally, people are interested in certifications in order to improve their chances in the job market. Employers are interested in finding the most qualified candidates based on their specific needs. The more specific the needs, the more likely a certification might be the best option. 

A college Computer Science degree demonstrates a strong and broad based understanding of technologies and general concepts, a great foundation in computing. It says little about knowledge of a specific version of let's say, an Oracle database. College graduates, while having a good base, may have little knowledge of the specific technologies and need additional training to be brought up to speed. This means for jobs that require a broad range of skills a college degree may be the direction an employer chooses to go, but what about the reverse? In those situations where a very specific skillset is needed, often a certificatied candidate will have the leg up. The first step in this case is to research the type of job you're interested in attaining and check the minimum requirements. Do they ask for a degree? The they ask for certification? A specific one? Looking at a variety of job postings in your desired field will give you a feel for what is expected.

After you've determined that certification is a valid method for achieving your goal, the decision becomes which certification, and how to obtain it. If your research uncovered job postings requesting a specific certification, say an Microsoft Certified Systems Engineer (MCSE), that makes the choice easy. In these cases it's best to work towards the specified credential in order to have the best shot in your chosen area. Many times these other options, a college degree or program completion or a traditional certification will be the best answer. 

Then when should you use an online certification? An online certification, such as those offered by the National Computer Science Academy (NCSA), can be a very valuable tool in many situations. Oftentimes jobs across a wide spectrum of fields want candidates with basic computing skills such as Microsoft Office, HTML, and just general ability to deal with computers. If a candidate does not possess a degree or certification or provable experience then it's hard determine who should be given a second look. In these cases, an online certification may be the best way to break through the clutter and get your resume to the top of the stack. That's exactly why NCSA certifications were developed, to document basic computer skills for those who have experience, but little or no formal education or training, in a convenient and reasonably priced manner.