Juniperus horizontalis 'Wiltonii', aka Blue Rug juniper, is about as flat as you are going to get. Typically listed at a 4-6" height but could eventually get slightly higher.

If you can find it, J. horizontalis 'Pancake' is supposed to stay even shorter but I've not seen this in person.

Low forms of Juniperus communis.

Juniperus horizontalis 'Pancake' is way flatter than 'Wiltonii', actually. It is almost too flat, in that you have to be careful how you use it or it is overwhelmed by what is around it. It is like painting the ground blue. It is far and away the flattest of any that I have grown or seen, so GG you can now take that to the bank!

Icee Blue and Mother Lode are nice colorful cultivars

GG I have about 7-8 of them and they never have exceeded 1"! Except for maybe at the stalk, which might be as high as 2".

How hard are they to keep the weeds at bay? A good mulch?

Ken put a photo on here years ago and used his garden hose for scale to show how low 'Pancake' grows for him. Maybe he will repost it. Here's a couple photos w some gold 'Mother Lode'.

'Pancake' is a very appropriate name for this cultivar!!

Either that or 'tortilla'! And I don't get many weeds in mine - it sort of mulches itself!

How about 'Splat'? In addition to being apt it makes it sound like it's another German garden conifer introduction.

As most do, 'Mother Lode' becomes taller with age.

http://www.iselinursery.com/colorful-conifers/juniperus-juniper/juniperus-horizontalis-mother-lode/

Appearance of young specimens makes me think they should have called it something like 'Spillage' or 'Broken Egg'.

I see 'Pancake' is on the Iseli site also.

http://www.iselinursery.com/colorful-conifers/juniperus-juniper/juniperus-horizontalis-pancake/