Battlefield: Bad Company 2's unofficial mod support is better late than never
Tricksy modders have wrestled control of Battlefield: Bad Company 2's architecture from DICE's iron fist. Using... magic, probably, the community at Emulator Nexus have reverse engineered EA's servers, allowing support for custom matches that support modifications.
In layman's terms? Super powered helicopter rocket attacks are go.
Accessing modified servers simply requires downloading a tiny .dll file for your BC2 directory. Hosting one is vastly more complicated. Instructions are available here, for those who understand this sort of thing.
While Bad Company 2 is a bit long in the tooth, it seems Emulator Nexus' co-founder NoFaTe is also working on Battlefield 3 server emulation. It's still in the early stage, with no public release mentioned, but could potentially lead to user-hosted modified servers in that game too.