I've written quite a few extensions for AWS Elastic Beanstalk. They are SUPER extensibe and very powerful but some of the customizations are not well documented and not obvious. So, I've "annonomized" the ones I've created and posted them here in the hopes they may benefit others.
01_aws_resources.config
This extension shows how to customize existing beanstalk resources (load balancer, security group) and add a new resources (SQS queue). It also shows a technique to get the auto-generated queue name onto the instance. For the load balancer, we have a beanstalk parameter set called HTTP_SERVER_PORT which we are then substituting into the security group rules.
01_install_s3get.config
This extension installs a handy python script I found which allows GET of content from a private S3 bucket. The nice thing is that using instance roles, it requires no access keys and the role can be used to lock down access to only the bucket you need. It does nothing more than add the script/tool to an instance.
01_install_updateroute53.config
Installs a small utility that allows updating of 'this' instance's IP address into Route53. Takes a domain suffix as an argument (which in our case, we pass as a beanstalk paramter and then pass into the script later). Uses instance roles for access control.
01_papertrail.config
Extension to configure the instance to send logs to papertrail (http:https://papertrailapp.com). I based this on the article in the papertrail knowledgebase BUT I modified it to use a 'hostname' of the beanstalk environment name concatenated with the instance ID. Makes finding and searching the logs very easy.
01_tag_resources.config
Small extension which shows how to tag instances with custom tags.
05_set_hostname.config
We had a requirement to have a hostname that we control registered in DNS. This extension sets the hostname to -. It then adds on the domain suffix passed as beanstalk parameter called DOMAIN_SUFFIX. Finally, it calls the updateroute53 script installed by the 01_install_updateroute53 extension to update DNS.
10_cloudwatch.config
Generates custom metrics for sending to cloudwatch. In this example, the metric is a count of the number of processes running matching a particular pattern but it could be anything. We tag them against the instance ID so we can see the metrics next to the standard instance metrics like disk space and CPU.
02_elasticip_assigner.config
Auto-assigns an elastic IP to "this" instance. Complicated quite a bit by the fact that in EC2 classic, an instance can "steal" an EIP from another and there is a massive race condition. This all works pretty well with one small exception. Messages in an SQS queue (used as a lock mechanism here) are auto-removed after a max of 14 days. This extension relies on there always being 1 (and only 1) message in the queue. So we use a cron job to remove the message and re-post it once a day (the keepalive). The hole is if something happens (crash, etc.) between the delete and tha add, it will lock out all other instances from being able to obtain the lock from the queue. It's a small possibility but still there. This is the most complex extension I've had to create!