As of Sept. 18, 2013, Twitch now requires that you log into IRC using an OAuth token instead of your plain text password for additional security. 

 Use this app to generate an OAuth token to authenticate with Twitch IRC.
Once a token is generated for an account, it is verified and saved locally.
The entire OAuth token prefixed with "oauth:" can then be used as password in your IRC client.

The Twitch Chat OAuth Generator is especially useful for people with multiple Twitch accounts.

Note that each time you generate a token using the same account credentials (username/password) any existing chat token for that same account is invalidated. In other words, you can only have one valid token per twitch account.

You can also import valid OAuth tokens generated by other apps.

Download Twitch Chat OAuth Generator.

Twitch Chat OAuth Generator requires the Adobe Air runtime.