Free CDN

As the name suggests, CDN or Content Delivery network delivers your content to users but with more speed and performance than your server can provide.

CDNs consists of servers, also known as edge servers or POPs(point of presence), distributed all over the world that caches your data like images, videos, javascript, CSS, fonts etc and delivers it to that region.

That way the data delivers faster to the user and you get lesser load on your server and better experience for your users. 

Advantages:

• Cloudflare is completely free for basic package that is enough for most users.
• Additional services are also included like DDoS protection, flexible and free SSL, code optimization, firewall, DNS, analytics etc.
• Most of the services are free but few needs upgradation to paid plans.
• POPs located all over the world.
• No need to change your code. Only nameserver change required.
• Easy to use DNS service.

Disadvantages:

• Sometimes genuine users get blocked by cloudflare from reaching your site and they have to fill captcha to visit your site. That pisses them off.
• One or the other issue is always on their network. See  cloudflarestatus.com. You will see daily something goes wrong with their services. To be fair to them I must say in such huge network anything can go wrong anytime.
• As name servers need to be set to cloudflare so complete control goes in hands of cloudflare.
• I have personally faced too many issues with their service.
• If you use letsencrypt SSL certificates letsencrypt will not generate or renew certificate until you pause cloudflare.

Advantages:

• Incapsula is like cloudflare with less features.
• Basic free plan includes CDN.
• Additional services like analytics, code optimization and DDoS protection is included.
• More services are available if you upgrade to paid plans.
• No changes in code required. Only A record change and a CNAME record required.
• POPs located all over the world.
• Better CDN performance than cloudflare.

Disadvantages:

• Not extensive network like cloudflare but good.
• Requires changes in A record of domain that mean complete control of your site goes in hand of incapsula.
• If you use letsencrypt SSL certificates letsencrypt will not generate or renew certificate until you pause incapsula.

Advantages:

• It is not actually free but it give 100 GB of free CDN bandwidth with no time limit to use that data. Warpcache changed its free usage to 250GB for a month so its not of great use to most now.
• Enough bandwidth free for regular user, who are looking for free CDN to use, for long time.
• Warpcache is multi CDN network, that mean they use multiple CDN networks like edgecast, akamai, highwinds, maxcdn, hibernia etc to deliver your content.
• Multiple CDNs mean more pops and better speeds to user.

Disadvantages:

• You have to pay after the initial 100 GB free usage. 0.08 USD per GB after the initial 100 GB usage. They now offering 250GB with only one month validity and they charge for 1TB per month per host for minimum now.
• Requires changes in the code to use CDN.
• New company with product still in development.
• Few basic controls are missing at present.
• Only one site or zone can be added in free plan.

Advantages:

• Free for 1 year with monthly bandwidth of 50 GB and 2,000,000 http/https requests.
• Popular and reliable service.
• Very good Delivery network with pops all over the world.
• Advanced control panel with lots of features and controls.

Disadvantages:

• Not free after the trial period.
• Becomes too costly when using paid service.
• Code changes required to use cloudfront services.
• Complicated interface for beginners.

Cloudinary is image and video only CDN. So you will not be able to deliver css, js etc via it. Now you can also use cloudinary to deliver javascript and css too. But you will have to upload the files to cloudinary and replace links. Also free quota increased toActually it is more than a CDN. Its a image processing service which delivers processed images through CDN. They use multiple CDNs to deliver the images. Earlier they used akamai only but now they figure out which CDN will be better and use that CDN.

Advantages:

• Free quota is free for lifetime.
• Free quota can be raised by tweeting about the service and liking their facebook page.
• Image processing a added benefit.
• Good delivery network as they use akamai, cloudfront, fastly etc.
• Can deliver videos too.
• Nice control panel to upload images and videos directly.
• WordPress plugin available.

Disadvantages:

• No support for content other than images and videos.
• Free quota is only 5 GB per month. Cloudinary shifted to credits based pricing. Now you get more in free quota.
• Code change is required to use the service.
• Not very easy to use if you have large number of images already added to site.

Pros:

• Free 10 GB data per month for US and UK.
• Free 5 GB data per month for Rest of the World.
• 52 pops all over the world.
• Advanced controling option.
• Full site acceleration.
• Custom SSL certificate allowed.
• Server sheilding like cloudflare.
• No additional http/https request charges.
• Access log
• Wordpress plugin available.

Cons: 

• Documentation is not good.
• Confusing control panel.
• Pops in only 4 locations in Asia.

Imagekit is primarily a image optimisation, transformation and delivery service but it also act as complete CDN and you can deliver all the static content of your site using it. 

Pros:

• Free 20GB data transfer per month.
• Free image storage space.
• Unlimited requests and transformations.
• Advance control panel to upload images to imagekit.
• Wordpress plugin available.
• Non image assets can also be uploaded to the media library.
• Cache can be cleared from imagekit dashboard
• Good documentation.

Cons:

• No custom domain support in free plan.
• Paid plans are costly for small site owners but maybe you won't have an upgrade if site is so small.