How lightweight are you talking? Are you looking for a hosted tool (like fogbugz) or are you definitely going to manage/host it yourself? I've been using Jira,

How lightweight are you talking? Are you looking for a hosted tool (like fogbugz) or are you definitely going to manage/host it yourself? I've been using Jira, and there's a $10 version for small groups. Whatever you chose, make sure of one thing: every tracked bug or task has a unique URL.