12 great apps for bridging Windows, Linux and Macs

#6 PurplePixie Systems' FreeNATS: The name says it all -- the best of both worlds. A Network Automatic Testing System (NATS) that is open source (Free) and cross-platform.
#11 PresenterNet's PresenterNet: Web conferencing for Windows/Mac/Linux users with any browser, nothing to install.
#9 Zimbra's Collaboration Suite 5.0: A cross-platform e-mail and collaboration suite.
#7 Compiere's Compiere's 3.0: Open Source ERP, CRM, plus OSS & CSS and more acronyms than you can shake a stick at. Barrett claims Compiere is a great package to manage your business.
#2 xTuple's ERP: Provides cross-platform accounting, ERP and CRM tools for businesses based on a hybrid open source licensing scheme.
#12 Civil Engines Research's Civil Netizen: personal file transfer system to move files between cross-platform systems easily.
#5 SharedPlan Software's SharedPlan Pro: Simple project management software for a heterogeneous environment.
#8 xTuple OPENRPT: OPENRPT is an open source and cross platform database reporting tool. OPENRPT, comes as part of xTuple ERP (slide No. 3 in this slideshow) or as a standalone database reporting tool.
Windows, Mac, and Linux -- can't we all just get along?
#4 SolarWinds' Orion Network Performance Monitor: Real-time network performance and in-depth analytics for servers, routers and switches in your cross-platform networks.
#1 Likewise Software's Likewise Enterprise: Integrates Windows, Linux, Unix and MAC systems into Active Directory and provides hundreds of ready-made policies.
#10 Zope.org's Zope: An open source content management, intranet, portal and application server for your cross-platform environment.
#3 Wimba's Classroom 5.2: A cross-platform virtual classroom that combines audio, video, content display and application sharing.