The only inline heaters I've ever seen are the Hydor external heaters. I have been thinking about getting one, I just wish they had bigger connections (they have 5/8" barbed fittings).
I've used the Hydor ETH 300w inline heaters on my smaller tanks and they are dead accurate and very reliable. They're perfect for canister filters with 5/8id tubing.
Rainbow Lifeguard also makes a inline heater module that will fit 1'' or 3/4'' connections. Never tried them though.
RENA FOR THE WIN... YOU COULD TAKE A HAMMER TO IT... LITERALLY and your not breaking the shell... thing is sweet... cheep on dr. foster for 300w.. it's all i have heating my 140 and it is not on all the time.... you can tell by red led... it is inside the tank but filter pipe hooks right up to it... if your looking for outside the tank then hydor...