Most of the newbie’s to the web arena just look at the cheap aspect when registering domains. Yes, no harm in looking at getting cheap. Because you are getting the same domain name though any of the domain registrars. But things to consider are the customer support you would be get, whether the domain registrar will vanish after some time, pricing after first year and many other things.

Though every one likes to get things cheap, most of the time a hitch comes free with being too cheap. If any company is giving away .com registrations at a price tag of below $5.50 then most probably you get some hitch free of charge, such as you have to host the domain there at a high price or you pay a comparatively high price in after the first year or they own the domain you just lease it. (unless you read the agreement carefully you wont find these hitches)

Its not nice explaning cheap domain registration offers with hitches, without an example. Take Yahoo for example. They do offer .com for $1.99 for the first year. But when read through the offer details page which hardly anyone would go through it says you need to pay $34.95 per year after the initial term.  So if this is the way famous yahoo provides cheap domain registrations what explanations are needed for the not so famous places offering domains at a price lower than the standard. 

Rule of Thumb No 1
Check whether the domain registration company is ICANN accredited, which is the governing body for Domain Registrars.
There are loads of domain registration companies who just pop up like mushrooms but you will be amazed out of that how many are actually ICANN accredited registrars.

Rule of Thumb No 2
Go for a famous domain registrar which was there for a while

Rule of Thumb No 3
Try to avoid places which gives .com domain registrations cheaper than $5.50, as they have to cover the costs by some other means because it cost more than that for them.

After comparing pricing, customer service, Accreditations and many other aspects would recommend GoDaddy as the best place to register domains.

If you are from US you should have definitely heard about GoDaddy from their TV ads. They do sponsor the famous super bowl as well.

Use Zine3 as the promo code and you get .com registrations for $6.95 which is a 30% discount over the standard pricing of $9.99. This works for renewals and transfers as well.

Few more coupons at GoDaddy
gdp0902a - 25% off on .com, .net, .org, .biz, .name, .us and .ws registrations, renewals and transfers
gdp0828a - private registrations $3.99
gds0124a - save 25% on $75 or over
cjchea30 - $5 off the order of $30 or more
gdp0916a - .Tv $15.99
gdp0915a - .mobi renewals $9.99

Apart from the overall best GoDaddy these places are also recommended as they have some cutting edge advantage in some areas than GoDaddy.

Moniker.com
One of the most trusted places among domain investors. If you are into investing in domain names as a business rather than just registering few for your business this is the place to go for.  Offers .com at $7.29

NameCheap.com
Offers free whois guard. Nice and easy to use interface if you are managing a big domain portfolio. use coupon code GLOWINGSKY to get .com domains at $8.41 (standard price $9.29).

Name.com
Accepts unverified paypal accounts. Offers .com registrations at $7.99. Allows to register one .net at $5.99

DynaDot.com
Accepts MoneyBookers. Most are extremely pleased with their customer service. Offers .com at $8.99 At the moment DynaDot offers IDN .com registrations at $6.50   

There are few more places which can be recommended but these are the best and big places to get a .com in terms of pricing, security, and customer service.