Price: $0.12
Pros: If you listen to the advice given on the website then your image should come out great! You have to keep it relatively simple with not too many details. Great pricing too when you really consider what kind of customization is being done. Low shipping cost as well. Can have a sample done before making a full order. Complete outside of the cap is white except for where your image is of course. Says they are working on including other colors to choose from as well for the base color.
Cons: It took a little over 2 weeks for my caps to finally ship out, but I think that had to do with the holidays around the corner. Their website told me that there was currently a high volume and they were experiencing delays.
Recommended? Yes

Comments: Here is a pic of my caps in person. Sorry for the bad quality (cell phone) and the glare from the lighting.

Price: $9.99
Pros: Easy Real cheap Awesome Caps!
Cons: minimal cap damage during shipping
Recommended? Yes

Comments: This company is great. For about a year before they started I was looking for affordable custom caps without luck. They are the only site that can make your cap design at a reasonable price, trust me. Ive never had my caps that were below expectations as far as quality goes. They have great customer service to boot. Once, I sent an email telling them my bottling date was approaching faster than expected. They responded quickly and had my order out the next day!

Like any start up company, they had a few kinks to work out. Damage during shipping was a big one but they have since came up with solutions to solve this.

Price: $0.12
Pros: Great for low volume runs, and amazingly fast turnaround.
Cons: None really.
Recommended? Yes

Comments: Contrary to kahlerbrew, I would have to say that the caps came out amazing. I had a relatively complex logo (shizmobrewery.com) printed and it looks amazing.

Will definitely be using these guys for all my small runs.

The online design tool is super easy to use, but it is very basic and doesnt let you do any image editing. You can resize and center, but for the image itself, you'll need to use an outside program.

Price: $0.12
Pros: Easy interface to use Good customer service The only service of it's kind (that I know of)
Cons: Mediocre print quality (digital printing) Printing on caps is prone to damage in shipping
Recommended? Yes

Comments: Bottlemark's online interface is super easy to use, and they responded and shipped my order quickly – This is the only service of it's kind that I know of, and I've been looking for something like this for years. I'd have given them 5 stars if it weren't for the cons...

The printing on caps is prone to scratching off and getting mucked up. It's digital printing, so the quality isn't all that great (not crisp like screen printing), but if you keep the design super simple, it should turn out okay. About 40% of my caps arrived with imperfections or scratches (probably from rubbing against one another during shipping), so I'm going to contact the owner and see if they'll replace the damaged ones... I'll write a follow up review after that.