https://github.com/joshlang/Secp256k1.NET - A managed (.NET) wrapper around Sipa's secp256k1 implementation.
I only threw it together a bit more than an hour ago, so obviously I haven't thoroughly tested it

But so far I've verified blocks up to 77700 with it and its working just fine. 78100 now. 78200! ...Geez it's fast

SIPA = INCREDIBLE.
If anyone finds any issues or has questions, feel free to get in touch.
EDIT: HOLY MOLY. 100,000 VERIFICATIONS in 9.8 SECONDS!
EDIT AGAIN: ...ON A SINGLE PROCESSOR!(@!1(&!(!(@!$(&#@!%
EDIT ONCE MORE: ...Doh, I think it caches results. That test was verifying the same signature over and over again. On the blockchain, signature verifications are taking about 10ms on average. Still 100x faster than Bouncy.
EDIT FOR LIFE LIBERTY AND HAPPINESS: Without a debugger attached, I'm easily getting <1ms per verification with Sipa
