I still wish they had included a few analog inputs and outputs to make it easier to use with sensors. Without that, you basically have to connect it to an Arduino or similar to do anything interesting. It seems kind of wasteful to use the Arduino just for sampling the analog signals and forwarding the data to a more powerful ARM board.
You can use the gpi bits to do A->D and D->A, you could use a USB sound device as analog input (but you are constrained in sample rate choices), in some cases if you strip the input capacitors you'll have level reading capability.
The cheapest way to get A/D is to add a bunch of resistors via a summing network to one input of an op-amp and the other input to the signal you want to measure. The output of the op-amp goes into one of the digital inputs. Binary search across the output bit combinations to find the point where the input bit swings. This you can do for pennies.
Lots of options to add analog without having to add an arduino.