I just came across a really useful primer on KPIs for SaaS companies here (thanks John Coldicutt at Fizzback for the link). It's a piece by Joel York linking KPIs to the B2B customer acquisition process.
There is no shortage of sources online for customer acquisition KPIs, mostly designed for the consumer Internet, although we are starting to see more science around B2B customer acquisition too. What I like about Joel York's post is that it applies the language of process optimisation (bottlenecks, resources, critical paths, etc) to figuring out which KPIs matter to your business.
This is much more useful to B2B companies who understand their processes, but have struggled to fit square KPIs from the consumer Web into round process holes.