Accepting 0-confirmation transactions and detecting double-spends with open source solution
You should not interact with bitcoind, because it (current versions at least) filteres double-spend transactions

You should have your own network-sniffer solutution - the program which connects to the network nodes (as many as possible) and listens for all transactions, checking do they have conflicts with particular ones.

(How much would you pay for such program? Just interesting :) )

This is also not 100% safe solution - a miner can place double-spending transaction in block and mine it himself. Look here: https://bitcointalk.org/index.php?topic=327767.0