For both purposes Microsoft offers freely available tools:
For inventory and assessment of your infrastructure you can use
MAP toolkit, which works will almost all editions and version of Windows Desktop & Server OSs using WMI, and moreover it also can inventory HP-UX, VMWare ESX(i) and a variety of Linux distros.
MAP can also gather data from multiple domains as long as the appropriate administrator credentials are provided.
Perhaps, worth mentioning it can collect MS SQL/Exchange/etc specific data.
Microsoft Assessment and Planning Toolkit
For patching purposes as guys previously mentioned above MS offers
WSUS, which is free as well. Will help you with patch management, allows create separate groups, like test and prod, deployment rules etc.
On the other hand we've been using
VMware vCenter Protect(i.e. Shavlik) recently on a number of clients networks, which is not free, but have grace period. There are also different editions. Shavlik allows not only patch management, but also offline VM patching, power management, asset inventory and etc.
VMware vCenter Protect Editions