How can I be alerted when an EBS instance is running out of space?

I agree with Frederick Cheung's answer; however, I found this utility for Windows instances: http://www.eleven41.com/labs/cloudwatch-monitor-for-windows/. It runs as a service and will send the disk space and memory usage metrics to CloudWatch for you. From there, you'd just need to set up the alarm in CloudWatch.

There are no built in metrics for disk space - Because Amazon is looking at this 'from the outside', they don't know what you've done with an EBS volume: it could be part of a raid set, formatted in some exotic format etc.

Amazon has a system called CloudWatch that can be used to monitor AWS systems and resources. Luckily, CloudWatch supports custom metrics: you can use the CloudWatch APIs to add any data you want (and then create alerts off them).

Amazon provides some sample scripts that will publish (among other things) disk space utilisation to Cloudwatch. Do be aware that there is a small charge associated with using custom metrics - $0.50 per metric per month.

Selected answer will no longer work because:

The CloudWatch monitoring scripts are deprecated. We recommend that you use the CloudWatch agent to collect metrics and logs. - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/mon-scripts.html

I found this guide helpful at setting up custom cloudwatch metric such as disk monitoring: https://marbot.io/blog/monitoring-ec2-disk-usage.html

The only thing that the above is missing is setting up IAM role and attaching it to the EC2, guide for this is here (do this first, then follow the blog post): https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/create-iam-roles-for-cloudwatch-agent.html

Per recommendation, summary steps below:

1. Set up IAM role per link here: https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/create-iam-roles-for-cloudwatch-agent.html
2. Attach it to EC2 you want to monitor
3. ssh into your EC2
4. cd /tmp
5. wget https://s3.amazonaws.com/amazoncloudwatch-agent/amazon_linux/amd64/latest/amazon-cloudwatch-agent.rpm (to download the agent rpm)
6. sudo rpm -U amazon-cloudwatch-agent.rpm (to install)
7. sudo vi /opt/aws/amazon-cloudwatch-agent/etc/amazon-cloudwatch-agent.json (to edit config)
8. paste the config (in the link, no changes necessary)
9. sudo systemctl restart amazon-cloudwatch-agent (restart agent)
10. tail -f /opt/aws/amazon-cloudwatch-agent/logs/amazon-cloudwatch-agent.log (see if its working)
11. Navigate to cloudwatch console and set up alarm based on custom metric that will show up as CWAgent (thats the name specified in the config, more detail in the link on exact way to set up alarm)