I have a Swissflow SF800. It's 50 euros, and hooks more or less straight to a fan header on your motherboard. It is reliable, extremely long lasting, has very low flow resistance and get this: it is very accurate. It emits 100 pulses per second per litre flow and its range is from 0.3 to 20 litres per minute.
I hooked one up to a fan header, told MBM to divide the signal by six, and presto: flow in millilitres right there on my desktop. If I feed the data into Samurize it will display a pretty graph of my choice on the desktop, and litres per minute to the third decimal if I want it. No fuss.
It's tiny and comes with 3/8" BSP thread on either end so you just screw it into your existing setup (well, I did anyway). It's the black thing on the Aquatube with the cable coming out:
If you want a flow switch, Gentech does one with 3/8" BSP connections that operates with a (weak) spring-loaded valve:
As such it can be mounted in any position. But if you don't want any moving parts at all, Gentech does this little black oblong box, again with 3/8" BSP connections, which measures flow through ultrasound.