Rockbox

God! My vote for realtime effects!
Not sure about how much processing can support the cpu in the H300 but probably some basic effects (like distortion, delay, flanger) could be done.

A note for developpers, on this website there’s some source code for audio effects:
http://www.musicdsp.org/

I second this one!

To be able to plug a guitar into the line input of a rockbox device, and hear/record an amp simulated signal, would be very cool indeed, especially if paired with multitracking capabilities (see my and others rant on http://www.rockbox.org/tracker/task/4956 ).

The most needed features of an amp simulator plugin would be

1 the amp simulator itself , perhaps with a choice of a few models.
2 an input volume knob, aka “drive” 3 basic tone controls. Either just a simple tone control, or perhaps one for bass, mid and treble
4 cabinet simulation - very important in order to sound like an actual guitar amp
5 all other plugins, reverbs, delays, phasers, flangers, autowhas and effects we can whip up

Of these, no. 1 and 4 can be combined. After all, they’re both about changing the input signal. No. 3 could be of relatively simple layout. No. 5 is just a bonus, and not really that important (although cool).