Can I volunteer as a developer for a startup?

I would recommend post to hacker news with your offer.

http://news.ycombinator.com/

It may not be as easy as you think. Companies may be wary of you because you can leave any time and they may be required by law to pay minimum wages or be affected by other laws.

I think you are underselling yourself and you could get a paying job. If you want to volunteer look for a charity/not-for-profit that needs technical help. Your goals should be learning how to communicate with people, being part of a team etc (not learning more programming) so you will be better at interviews. Remember - you impressed them enough to get to the interview stage.

And don't be scared of the business side, you might surprise yourself and want to start your own business.

Can I volunteer as a developer for a startup?

You could volunteer, but you should ask for equity instead. Most likely the startup will go nowhere and your equity will be worthless, but that's how things are done in business.

I am pretty good with Linux, .net, Jquery and PHP. Not saying I'm the worlds best but if I'm told what to do I get it done in reasonable time.

It sounds like you have some solid skills but either aren't familiar with the professional software world or haven't yet found your niche. The thing you need to do is start meeting people face-to-face. Back in the dark ages, people dressed up in suits and went to job fairs. Nowdays the technology world is so mainstream that you can often find relevant local meetups that offer technical discussions and sometimes free food. Pick subjects that you are comfortable with, and you will find your social anxiety melts away quickly. Dress casually, no one wears suits anymore.

I do some stuff on my own but I'm jumping from thing to thing and never get a solid project done. I'm desperate for guidance on good idea.

Find a mentor. Find someone technical who you respect and has a solid track record of completing projects. Ask to setup a mentorship relationship. Meet every week if you can, or every month if that's the best you can get. Get advice on how to complete projects. Then start completing projects.

If you're up for a short bit of travel for a bit, I recommend checking out http://www.startupify.me/ - the founder behind it is taking in another cohort of developers, and it would give you some work experience and some time to experience some startup work too.

I know the space and founder - highly recommended.

The biggest concern I have when I see someone with experience offering to work for free or for a speculative share is that you attract the attention of a lot of individuals with whom you should really not be dealing.

I mean that those who are seeking to take advantage of someone are constantly sniffing out opportunities to get free labor out of people.

I have had a couple of very raw deals in my past professional life in this vein. One of them was doing web development work for a very respected local chamber of commerce, in exchange for referrals. Everyone right to the top knew what I was doing for them and they did absolutely nothing to help my business. In short, they seemed to regard my help as kind of a joke. And this was a "pro business" organization.

If it is a commercial activity, ask for some kind of compensation. Even for a non profit, be intensely critical of their actual need. Free work usually isn't respected by anyone. The only situation in which you should work "for free" is that you are a partner with undilutable equity.

Equity alone? Bah. At least get paid something - and ONLY for experience in new technologies or domains that will help you later.

If a venture can't afford to pay for work, I question its "legs".