In the past I've been playing around with RS232 and found it quick and simple to use from both the interface from both the FPGA design and the client software, so I sketched out a design and over a couple of nights CheepScope was born!
* 16 channels
* 1024 samples
* Requires only one I/O pin - or no pins if your Dev board has a USB to RS232 interface
* Light on resources - one Block RAM and about 100 slices.
I've made a very simple character mode user interface for use under Linux. It is functional but not flash. It is not as good as the Vendor's VLA as it needs to be added to your design it is built, but it works a treat.
It's all up on my Wiki at http://hamsterworks.....php/CheapScope
If (like me) you don't run Linux natively you can always use Virtual Box's USB device pass-through, allowing your VM to see the USB-to-Serial device.