Stating the obvious here, but it's been very helpful for my training as a poor man's PowerMeter / Computrainer:
Cycle Ops 2 Fluid Trainer + Bike Computer with speed (+HR) sensor. Obviously not translatable directly to road use, but you can get a proportional estimation of power by using the wheel speed. The first few times I did this for 60-90 minutes, I was surprised at how badly my speed/power was dropping off at the end of the ride - with no coasting or stopping on the trainer, you really get a more honest workout. For me to hold a steady speed/power on the trainer, the first 15 minutes tends to feel surprisingly easy and doable, where as minutes 75-90 have me working pretty hard.
This setup is also great for interval work, to make sure you're being steady on the intervals. Costs a lot less than a PM or computrainer as well!