Sequel is being actively developed. New versions of Sequel are generally released monthly during the first week of the month. You can join in on the discussions, ask questions, suggest features, and discuss Sequel in general by joining our Google Group - Sequel Talk or our IRC channel.
The master source code repository is jeremyevans/sequel on github. The latest release version also has a git clone at RubyForge.
The easiest way to contribute is to use git, post the changes to a public repository, and send a pull request, either via github, the google group, or IRC. Posting patches to the bug tracker or the Google Group works fine as well.
Sequel is distributed under the MIT License. Patches are assumed to be submitted under the same license as Sequel.