Here are the plugins I couldn’t live without:
Timesheet report – Summarizes any user’s time spent over a weekly period: http://confluence.atlassian.com/display/JIRAEXT/Timesheet+report+and+portlet
Charting plugin – Generate charts from any issue navigator: http://confluence.atlassian.com/display/JIRAEXT/JIRA+Charting+Plugin
Subversion plugin – Link and view subversion commit history to a JIRA issue: http://confluence.atlassian.com/display/JIRAEXT/JIRA+Subversion+plugin
Labels plugin – Lets you tag any JIRA issue and search on tags: http://confluence.atlassian.com/display/JIRAEXT/JIRA+Labels+Plugin