During the counting of the Omer we’re supposed to be in a state of mourning, since we haven’t yet received the Torah. Some people don’t go to live entertainment in the weeks between Pesach and Shavuot. Many won’t listen to music made with instruments, because that’s also prohibited during times of mourning. But you can still listen to a cappella music.
Be Honest: You love these guys
I’m not big on all the rules about what you can and cannot do during the Omer, but I do try to listen to some Jewish a cappella, because there’s so many great groups out there. Here are some of my favorites:

Pizmon- college group from Barnard/Columbia/JTS, they’re the pioneers of the Jewish a cappella revival. Coed, cute and funny.

The University of Pennsylvania Shabbatones are almost frighteningly talented. I have their last CD, Challah Back, and it blows my mind with every listen. Best tracks are Darkenu and Gam Ki Elech.

Kaskeset from SUNY Binghamton have gorgeous voices, and gets props from me for their name. Kaskeset is the Hebrew word for fish scales used in the Torah. Get it? Scales?

Manginah from Brandeis. It makes sense that a school with so many Jews would have a great Jewish a cappella group, and they do. I prefer Manginah, but there’s also the all-male group, Jewish Fella A Cappella.

Kol Ish- This is an all-male group from University of Maryland. The name is a pun on a rule about men not being able to hear women sing, which I find kind of obnoxious (the pun, but actually the rule, too). Anyway, even though they kind of make me crazy, they’re really freaking good.

The Vokols from University of Pittsburgh are new and spectacular. I’m totally biased because my sister is a founding member, but whatever. They’re awesome. Check out the video.

There are also a bunch of Best of Jewish A Cappella albums available for purchase online.

And if you really want to rock out to some Jewish a cappella then you should probably get yourself a copy of the Miami Boys Choir Around the Campfire album. Now you’re cool.