I've tried most of the low hanging fruit for notation apps on Linux
and the best one I've used so far is:
http://www.musescore.org/
Lilypond is good for doing touch-up work or adding symbols Musescore
can't handle
caveat: the front ends available for LP leave a lot to be desired
and unless you like writing scores in a markup language (see below) then
I suggest something like Musescore
Lilypond example:
\relative c'' {
\key g \major
\time 6/8
d4 b8 c4 a8 d4 b8 g4
}