It depends upon what you're planning on studying and which online college you attend.
For instance, I am an online student and am studying medical transcription. This is a very difficult field to break into because most of the work available is in the work-from-your-home format. In my case, I researched out the local colleges for prices and curriculum.
Additionally, I researched out the medical transcription field thoroughly. Ironically, the top two schools in the nation for medical transcription ARE online schools. I chose Andrews School because I wanted their prestigious name on my resume and because their students always have multiple job offers that waive the 2-8 years of experience needed to get a job. In this case, the online school graduates are preferred and are in very high demand--and that demand is not being met.
One thing I did learn in my search for education that involves the online schools--beware of matchbook schools. Remember the old-fashioned matchbooks that had ads printed on the inside cover? You could get a degree in this or that...but you could never get a job with that diploma. (Matchbooks: Stratford, Phoenix, etc.)
You can find more matchbook schools by looking in the informational racks at the local supermarkets. Beware of these types of schools. They "teach" everything under the sun: accounting, veterinary science, massage therapy, sewing, legal aide, etc. Anybody can enroll, and they have such poor standards that everybody passes no matter how crappy their work is. They will always take your money. Sure, you'll easily finish the course--probably with outstanding grades--but the likelihood of you ever getting any type of employment from this type of school is nil. Don't waste your money.
Now, there are good brick and mortar colleges that also have online schools, and they award diplomas that do not necessarily mention the online training. You can even go to the cap and gown ceremony with the good schools.
You may want to take just one online course to get the feel for it. It is a very difficult way to attend college. You have to be the go-getter here. You have to schedule your hours and stick to those hours. It's not as easy as a lot of people think it is.
I've attended traditional brick and mortar colleges, and I've done regular online colleges. I will say this about the experience: depending upon your ambition, your tenacity, and your quest for knowledge, you can get a better education via the online option.
I have found that online college work is much more difficult than attending regular classes, and I often found myself wishing that I went to the brick and mortar college because those classes tend to be much easier.
Bottom line: research thoroughly before investing your money. Some occupations (such as medical transcription) actually prefer the online graduates. Other occupations, I suspect, may look down on folks who went the online option.
As far as which online colleges are credible, I can't help you there. You don't mention what you want to study, what area of the country your are located (some schools require you going to the campus for proctored exams), and what type of employment you desire.
Start by checking out the websites of the colleges and universities in your area. You can call the local schools and talk to one of their counselors and recruiters too. You're more likely to get a straight answer from them.
I almost enrolled in Kaplan, but I am glad I didn't. After I signed up on the internet for more information (they all require your phone number), I got dozens of dunning phone calls. Beware of the big push and the "sign up now" lest you have to pay more tuition routine. The matchbook schools employ telemarketers that have to meet quotas and get bonuses for each person (fish) they snag with a binding contract.
Do your research here. There are many excellent online schools where you can get a legitimate diploma that doesn't say "online" school. It just lists the name of the university or college.
Good luck with your search for a school!