People have set up web sites for Sirius where they log every song playing. So you can look at a grid and see everything now playing. I think they do it by monitoring radios that have a USB connection. As usual, the peeps are ahead of the actual companies in providing info online. They also built a better online player for the channels that are streamed, and a song search tool. Here's one grid, there are others I think: http://www.itsonsirius.net/
I haven't kept up with the latest, but this site used to have it for Sirius. Checkout the stuff at the bottom on the knowledgebase, under "Digital Expert Network":
But by far the most up to date info is in the forums, not the wiki, same site.
Another site I used to visit... http://www.dogstarradio.com/
And this one comes up on Quantcast*... http://siriusbuzz.com/
For XM the main place to go is http://xmfan.com/
... maybe http://www.xm411.com/
Or check out what the actual company websites have these days.
covers both XM and Sirius.
*Quantcast. Seems like an interesting way to find links. Plug in one URL, and it gives you the most popular related URL's. Ad cookie data mining at work, I guess... http://www.quantcast.com/
... the stripped down version I stumbled on is here... http://22.214.171.124/