OpenComputers interfaces with a lot of different mods, and many mods make use of OpenComputers' API, too. Here is a list of mods that are either supported by OC or support OC. No guarantee for completeness or correctness.
OpenComputers supports a good number of power systems.
OpenComputers supports the major bundled and wireless redstone systems.
OpenComputers can interface with the following mods to send network messages.
OpenComputers supports the following systems for covering cables.
A couple of mods support OC on their own, allowing computers to interact with some of their blocks, for example.
There are numerous addons that add blocks and items or drivers (support for other mods' blocks) specifically designed to work with OpenComputers. Please see the according section on the forums and the Addon page on this wiki for more information!
What was previously the OpenComponents addon is built into OC as of version 1.4. There are many drivers for the Adapter block to allow interfacing blocks from a number of other mods. It also allows using most of CC's peripherals, if CC is installed.