For me, unless I know exactly what package I want--and I often don't--I typically turn to Synaptic, the graphical package manager that graces my Ubuntu notebook. Synaptic is a really nice application, and I've spent untold hours nerdily sifting through the massive software catalog that Ubuntu inherits from Debian.
There are six packages in the repositories that match a search for "Software Defined Radio." They hail from the GNU Radio project, and it pleases me to know that when I finally get around to playing with SDR and GNU Radio, they're waiting just a few clicks away.
Like I said, nerdily.
I've mentioned favorably Ubuntu's simpler Add/Remove Applications tool in reviews before, but I don't usually use it myself, since I think of myself as a Power User.
It turns out, though, that the simpler tool launches faster and searches faster, too. I needed a color picker to help me come up with a color hex with which to customize a Web application. Just after I kicked off a search for "color picker" in Synaptic, I flipped over to the Add/Remove program, typed in the same search and still beat good old Synaptic.
So even us Power Users (circa Windows 2000) can learn to benefit from simpler tools--at least some of the time.
And happily, during the brief link hunt that I carried out to come up with the apt-get et al howto links from the first paragraph, I learned a couple new terminal tricks. I guess I'm off to play with apt-cache policy now, and inch imperceptibly toward command line gurudom. Nerdily.