There has been a lot of interest in CW and CW practice within the group. Here are some online resource that I have found and have used to work on my CW proficiency (or lack thereof):

Learn CW Online:http://lcwo.net

This is a free website that features an entire koch-method course, as well as practice qso's word groups, random groups, callsign training. You can download mp3 practice files to put on your mp3 player as well. The site keeps stats that visualize your progress. It is a very effective tool. It is free to use.

G4FON koch trainer: http://www.g4fon.net/

This is a piece of software that runs on windows that utilizes the Koch method of teach CW. The koch method emphasis learning code at a usefull speed, where the mind recognizes the entire sound of a character instead of breaking it into dits and dahs. The makes learning code much faster and easier, without the speed plateus that are encountered when learning code at slow speed, and then trying to speed up.

Use of these tools, along with on-air practice sessions can help develop your CW skill quickly.

Post other resource you are aware of as comments to this post.