Phoenix is the spiritual successor to the Protostar challenges. It covers the following topics:
The idea is to introduce the simplest concepts first, from memory corruption, modification, function redirection, and eventually, executing shellcode. These challenges are available for both 32 bit, and 64 bit mode. The 64 bit challenges are new, and were not available on Protostar.
You may download Phoenix from the downloads page.
For more information on how to get started, see the getting started page.