Some of the best extensions for Integration Services (and SQL Server itself) are found in what Microsoft has released as "Feature Packs".  They aren't very well advertised, but include some of the essential tools for certain scenarios - like the DB2 connectors, PowerPivot, and Report Builder.  Most of those tools are available individually (somewhere) but the feature pack is a good place to find them all.
Microsoft recently released the first SQL 2008 R2 feature pack (for RTM) - the link is below along with links to the most recent feature packs for the other recent SQL versions.
Feature Pack for SQL Server 2005
Download here. Integration Services relevant bits:
Microsoft OLEDB Provider for DB2
Microsoft SQL Server 2000 DTS Designer Components
Microsoft SQL Server 2005 Backward Compatibility Components
Microsoft SQL Server 2005 JDBC Driver
Microsoft .NET Data Provider for mySAP Business Suite, Preview Version
Feature Pack for SQL Server 2008
Download here. Integration Services relevant bits:
Microsoft SQL Server 2008 Analysis Services 10.0 OLE DB Provider
Microsoft SQL Server 2005 Backward Compatibility Components
Microsoft Connector 1.0 for SAP BI
Microsoft SQL Server Driver for PHP 1.0
Microsoft SQL Server 2005 JDBC Driver 1.2
Microsoft OLEDB Provider for DB2
Feature Pack for SQL Server 2008 R2
Download here.  Integration Services relevant bits:
Microsoft® SQL Server® JDBC Driver 3.0
Microsoft® Connect 1.1 for SAP BW for SQL Server® 2008 R2
Microsoft® OLEDB Provider for DB2
Microsoft® SQL Server® Driver for PHP 1.1
Other Addons for Integration Services
Of course, if you're looking for addons for SSIS, the place to go is the SSIS Community Tasks and Components directory on CodePlex.