This page will host some original content offering useful advice, and perspectives on developing musical ability. Until that's here, this will mostly be links to musician's resources and inspirational quotes and articles. For details of actual musicians I think you should investigate, see the links page.
I may expand the scope of this, as time progresses, but for now, that gives me plenty to be doing. In simple summary, my musical advice is to practise little and often, and to really focus, even if only for thirty seconds, as this is more beneficial than an hour of distracted twiddling.
To borrow from the collective wisdom; the answer is practise. Now, what is the question?
Guitar Cardio is a fanastic resource for the diligent guitarist. It constructs fresh drill exercises customised to your specific choices of scale, mode and so on. This is particularly valuable as it is so easy to subconciously choose to practise that which you're already good at, which obviously is of less benefit than something that truly pushes you.
Trainear, Jimmy Ruska's interval training software, is incredibly useful, whatever instrument you play, as it develops your interval recognition, which will hugely improve your musicallity, as well as helping you to figure out solos by ear and so on. As the natural goal as a developing musician is, I think, to be able to reproduce whatever musical idea pops into your head with your instrument, any tool that improves your understanding of what a musical idea is will be tremendously useful. Even if you already have developed interval recognition, or think this isn't for you, I recommend watching his explanatory video, as his summary of the development of musical notation is very amusing. He has his own website, JimmyR.com, which is a really useful news aggregator, amongst other things. Check it out.
Richard Lloyd, the guitarist from the seminal 70's punk rock band, Television, has some really useful explanations on his Q&A page. The question regarding tuning a guitar is particularly enlightening, and reassuring, if you've been having trouble (i.e.-it's not your fault!).
Justin Sandercoe offers arguably the most comprehensive range of free resources for the aspiring guitarist on the net. He also seems to be a thoroughly nice bloke. His advice about transcribing songs yourself rather than using tab sites is spot on.
Thelonious Monk's advice. (This scanned image will open in a new tab or window, depending on your browser settings.)
A note about courage by Jeff Buckley. (Again, this will open in a new tab or window, depending on your browser settings.)
Please do email me any suggestions you have to add to this list. I'm particularly aware of the guitar focus at present, so anything to redress that would be greatly appreciated.