By default CRM 2011 provides Autonumber solution for Contracts, Cases, Articles, Quotes, Orders, Invoices and Campaigns only. There is nothing such for the rest of the System or Custom Entities. The only possible way is the Plugins.
In Codeplex there are number of solutions available. I’ve recently started using this Auto Number Solution and found it very flexible with the Auto Numbering Formats. The Installation, Configuration and various Format structures are easy and described very nicely.
Kindly share, If you are using any other Auto Number solutions.
Update** 28/Aug/13 -> There was a personalized demand of autonumber which is based on a certain condition with the Auto-Number Format set as a number with Left Padding with Digits(0) & attached to a Prefix. I wrote a plugin for the same in my Following Blog