SharePoint (WSS 3.0)
1. Mullivan’s scheduled reminder& alerts
http://www.codeproject.com/KB/sharepoint/SPScheduedReminders.aspx
2. Zip and download list items
http://www.codeproject.com/KB/sharepoint/SharepointZippedListItem.aspx
3. Cross-Site Lookup Column Add-on
http://tonybierman.blogspot.com/2008/07/free-custom-cross-site-lookup-column.html
4. Tip of the day
http://www.amrein.com/apps/page.asp?Q=5739
5. Smart tools
http://www.codeplex.com/smarttools
SharePoint Designer
1. Useful SharePoint Designer workflow activities
http://www.codeplex.com/SPDActivities