Xoscope is available for installation directly from Linux distribution (I used Ubuntu). Also it's easy to build it from sources.
Uses ALSA for sound card support. The maximum sampling frequency is 44.1 kHz.
Most of the controls are nicely mapped to the keyboard keys, very easy in practical use - switching channels, timing, scaling, saving into memory, recalling.
Lightweight in resources use, easily allowed real-time processing of signals from a 3-oscillator software synthesizer running on the same laptop routed through JACK.
I wish there was also a built-in spectrum analyzer (as is the xoscope only measures the main frequency, it does not have ability to show the spectrum).