ec2id is a simple web application to display instance metadata from a running EC2 instance.
For more information on AWS Instance Metadata read the documentation
EC2 allows you to specify upto 16Kb of user data when launching an instance. The better classes of AMI (such as those provided by Alestic) will execute that user data as a script if it starts with #!. This allows you to launch ec2id when your instance starts.
Here's a sample script you could use:
#!/bin/bash
cd /var/tmp
wget http://ec2id.s3.amazonaws.com/ec2id.tar.bz2
tar fxj ec2id.tar.bz2
cd ec2id
python ec2id.py >/dev/null &