What's the Best Way to Attract New Users?

If they are looking at your website but not signing up, then I would look for problems with your website. It may not have a clear conversion channel, or may not instill enough confidence in the visitors.

If you are actually speaking with them in person, and they think its cool but don't sign up. Are you sure you have articulated the value to them? There is a difference between cool and seeing the value and wanting to use it.

You haven't provided a lot of information, so it is hard to offer more ideas. I guess your best option is to ask them? No harm in calling and saying "hey, we did a demo and you seemed to like it, but you haven't signed up. Would you mind providing any feedback why so that we can improve our offering".

What is the point of registering? If you want people to give your product a chance, then just let them give it a chance. It seems like you crossed an incredible milestone of getting users to even visit your site. This is HUGE. Take advantage of this and don't let them leave without trying your product.

This IMO, can be accomplished in several ways:

1. Introduce a video right on the front page of your product that tells a fun and brief story.
2. don't write an articles worth of content to describe your product. You could use clear, bold, large pictures/letters to tell a better story. This is a lesson Steve Jobs taught us.
3. Use Vimeo as your video host. Vimeo has a chromeless border that you can design around in your webdesign. Also, Vimeo is synonymous with creative, outside of the box, purple cow like and brain engaging.
4. Don't give desperate incentives to try the site. If users don't want to register then the only reason they'll register is to get the freebie. Everything they will have to go through to get to the prize will be seen as something annoying that gets in the way.
5. Try an age old tactic. When you were a kid your mother would make you eat your peas by masking the flavor with mashed potatoes. Mask your product in the same way. Mask it by making it better with something else. Make it a feature of something more grand. A good example of this is Skype. If you look at everything Skype is integrated to its easy to see that it's like Starbucks being on every corner of technology. Not knowing the product it would be impossible to make any specific recommendations. But have in mind how your product fits into the landscape with other products you can cross-integrate and do business with.
6. Give incentives to have people tell one sincere tweet about what they thought about your product. Ask them in a casual way. Say you will pick out the most sincere tweet to win a prize. A free month's membership, a free year's worth of senior level technical support. Something that will make a user who is passionate enough about your product to send a tweet about it - smile. If they participate awesome, if not they weren't interested anyway at this time. You'll have at least a few sincere opinions.

Check this post out on how to create a million dollar business: http://www.fourhourworkweek.com/blog/2011/09/24/how-to-create-a-million-dollar-business-this-weekend-examples-appsumo-mint-chihuahuas/

There's some great insight in there, such as how to test with Google Adwords to see just how interested people are. You can target in certain keywords and give a brief outline of what you're offering and put in an opt-in form. If people want to hear more, they can opt-in.
If the initial interest is there, then you can considering a free trial. Then it's a matter of continually asking questions and refining your product to meet your ideal customer's needs.