I like PowerTab a lot. It's free and pretty full featured but only inputs notes as fret placement. It shows notation, but you can't just click on the staff to add stuff; you always use the tab. I am okay with that for most purposes.
+1 on the powertab software. While it only lets you write the tab out, you do get to change everything as if you were actually writing the music score. It also allows you to choose from many different instrument sounds for your playback. The best thing I like about it is that I can compose both the guitar and bass score when writing a song on the same file and have both play back simultaneously. This feature is great as I use it to send my guitarist music I write, and it allows him to hear both parts and understand exactly what I am trying to accomplish. And if your into cover songs, there thousands of files for download on the net. I've got about 50,000 or so. I don't bother learning them of course. I just like to look at them and try to figure out what the bassist was thinking when he came up with his bass lines.
Presumably by "online" you mean something you can download. If you don't mind typing out your scores in a text editor (which is the easiest way to do most anything), then Lilypond will produce nice scores.
But if this is just for your reference, why not just write them?
Cheap/free notation program? 
