US
Aetna
Aetna is one of the biggest dental and health insurance companies. They have the largest nation wide network of approved dentists. So if you are looking for a HMO, start with them.
They have traditional policies tailored for many segments of society. They also have policies with a high level of customization available.
DentalPlans.com : Patriot Plan
This company's one plan, the Patriot Plan is worth mentioning. It is $104.95 per year for individuals and as low as $159.95 for families. Now you don't get that much cover for so little, but if you are on a tight budget and looking for minimal cover the you might want to look at this plan.Metlife
Metlife also has some high quality plans. Metlife is a large insurance industry player with life, home, auto ... insurance available. So if you want to do all your insurance through one company for simplicity, Metlife might be the company for you.
EHealthInsurance
EHealthInsurance is a large health insurance company with a few good speciality dental plans. They have indemnity as well as PPO dental insurance available. Their quotes are competitive so worth having a look at their site.
UK
Most people in the UK use NHS dentists. If you are, on the NHS, you may want dental insurance for the part of your dental fees that are not covered by them. If for whatever reason, you prefer a private dentist, the companies below will also insure you against that.
Tesco
Tesco is well known for high quality and low cost products. They have expanded into insurance and their policies are low cost and simple to use.
Money Supermarket
Money Supermarket is a insurance aggregator site. This means that they take your details, then based on that they suggest a few policies. You read the fine print on those policies and pick the best one.