OK so you've gotten your new receiver or SDR set up, got an audio connection going, but now you need to test to see if you're getting signals to your software.
Probably the best place to hunt for digital sigs just to get you started would be the ham bands. For those that aren't aware, the ham allocations are divided into digital/CW ranges, and voice ranges. You need to concentrate on the digital ranges, and allow for propagation changes for your time zone (it would be almost worthless to check a 75m frequency in the daytime, for example).
This link gives you a nice general overview of the amateur bands
In addition W1AW in Connecticut has regular transmissions
If you have questions about the digital signals in the amateur service, please post them here
Amateur Radio Data Transmissions - The RadioReference.com Forums
What about non-ham stuff? Well there are only a few schedules and here is what we have so far...
The NWS SITOR broadcasts - there used to be many more of them...
USCG HF SITOR
This guide to FAX station schedules (it's PDF) is a bit dated, but should still be mostly useable
And of course the regularly scheduled broadcasts every weekend from the VoA, thanks to my old friend Dr. Kim Andrew Elliott
Now the biggest challenge you'll face is trying to identify which mode to use. You can do some of this by ear - ALE and PSK31 sound completely different. We have a collection of links to various audio samples in our HF Decoding wiki
HF Digital Decoding - The RadioReference Wiki
I'm stickying this thread so we can accumulate other regularly-scheduled digital transmissions that can be used to test a receive setup.. Any schedules outside of the ham service