Official download pages:
TechCenter Office Communications Server 2007 R2 and Clients Downloads
TechCenter Office Communications Server and Client Downloads
Updates Resource Center for Office Communications Server 2007 R2 and Clients
Latest Cumulative Hotfix update for Office Communications Server 2007 R2 and Clients – CU9 June 2011

Deployment media:
Office Communications Server 2007 R2 Trial
Office Communications Server 2007 R2 Group Chat server
Office Communications Server 2007 R2 XMPP Gateway
Office Communications Server 2007 R2 UCMA 2.0 Speech Language Packs
Office Communications Server 2007 R2 Web Scheduler – see this post on how to install
Office Communications Server 2007 R2 Web Service Provider
Planning Tool for Microsoft Office Communications Server 2007 R2
Edge Planning Tool for Office Communications Server 2007 R2
Office Communications Server Public IM Connectivity Provisioning Guide
Office Communications Server Public IM Connectivity Provisioning Site
Microsoft Office Communications Server 2007 R2 Client Group Policy Documentation

Desktop client installation media:
Office Communicator 2007 R2 Trial
Office Communications Server 2007 R2 Attendant
Office Communications Server 2007 R2 Group Chat client
Office Live Meeting 2007 client and addin
Recording Converter for Microsoft Office Live Meeting 2007
Agent Communications Panel for Microsoft Dynamics CRM 4.0
Messenger for Mac 7.0.2

Mobile client installation media:
Office Communicator Mobile 2007 R2 (SmartPhone)
Office Communicator Mobile 2007 R2 (Pocket PC)
Office Communicator Phone Edition 2007 R2 (Tanjay)
To install directly on the cellphone and get java client, use http://getcomo.com

Testing and Training:
Office Communicator 2007 R2
Office Communications Server 2007 R2 Group Chat
Office Live Meeting Client for Office Communications Server 2007 R2
Unified Communications Adoption and Training Kit 2007 R2
Unified Communications How-To

Troubleshooting
Office Communications Server 2007 R2 Resource Kit Tools
Office Communications Server 2007 R2, 32 bit PreCallDiagnostic Resource Kit Tool
Office Communications Server 2007 Resource Kit Tools
Office Communications Server Netmon Parsers
Microsoft Network Monitor 3.3
PortQry Port Scanner Version 2.0
OCS Threshold Files for PAL

White Papers and Technical Documents:
Microsoft Office Communications Server 2007 R2 Technical Documentation
Microsoft Office Communications Server 2007 R2 Deploying Certificates White Paper
Microsoft Office Communications Server 2007 R2 Site Resiliency White Paper
Microsoft Office Communications Server 2007 R2 User Forest Topology
Microsoft Office Communications Server 2007 R2 Client Group Policy Documentation
Microsoft Office Communications Server 2007 R2 Workload Architecture Poster
Unified Communications Phones and Peripherals Datasheet
The UC White Paper
Configuring XMPP Gateway with Jabber XCP 5.4
Configuring XMPP Connectivity to Gmail
OCPE Device (Tanjay) Upgrade Guide
Microsoft Office Protocol Documents