There are a number of avenues to go for hiring, like LinkedIn, traditional Craigslist, Monster listings, etc.
I personally use Startuply.com (YC '08) for a lot of the hiring that I do. It's proven to be pretty good in terms of volume as well as quality, but you have to be pretty patient.
Where do you find candidates for hiring for job openings?
EDIT : Apart from networking. That's obviously the top for both quality and quantity.
When it comes to engineers, I strongly believe that open source projects are the best place to find candidates, for a few reasons:
And perhaps most importantly:
Personally I find it difficult to stomach hiring anyone on a salary of $80k per year or more, and generally for a good developer in the US that's usually the minimum you need to pay. So I highly recommend considering hiring globally working directly with talented people who are working from home. Even if you are hiring in the US consider hiring nation wide and working with team members remotely so that you are getting access to talent from across the entire country.
So consider hiring from our site www.Staff.com which is a global platform for recruitment focussed on full time hires only. Or other platforms such as oDesk.com (they do full time hires and project based work).
For US based hires I would look at:
Startuply - looks pretty good haven't tried them
I like the suggestion of looking for people that contribute to open source projects but have never been able to successfully contact these people (or at least enough of them to make a meaningful difference).
I really like the getting involved in the local educational community. Interns are a great way to find good people with great potential that can be molded for a start-up.
We are also getting involved with local universities to provide lectures / talks within our industry after finding that they are looking for people from industry to come in to give real-world experience reports on the subjects they are teaching.
Go out and network at events associated with your industry if they're available. By definition, people there are interested and experienced in your industry. Matching a face and physical interaction to an online profile and application can give you great context to if their personality and passions are aligned with yours.
github search for "relevant" projects, and contact authors
I've found that a majority of the time, those individuals who come through networking and personal recommendations are the best long-term performers. Exceptions certainly exist, but it's been almost eerily true over the past 5 years to see that folks we found via Craigslist or other job boards come and go, but networked/referred folks stay, contribute and perform at top levels.
I personally find LinkedIn to be a brilliant tool for this purpose, but your mileage may vary depending on the quality/quantity of your network.
Referrals from friends.
Linked in friends, and also filtering on reported people on concurs.
You need to keep in mind that the time you spend going though CVs and interviewing people is time you are not doing other things, which means that unless you are in an HR department, you need to be as efficient as you can, since you don't have the time to spend on going though as many candidates as possible,
Given that, I find that referrals from friends and employees work best, since they come with a recommendation to begin with, and they cost you no money. Posting online can be cheap or expensive, depending on the site, but either way, it will require going though lots of CVs with no recommendations.
If you are looking for a specialist that you cannot find in your network, then you have no choice but to spend the resources needed to advertise the job and filter the results.
Sometimes it might make sense to hire an intern/co-op student. Some are quite exceptional, cost less than full-time employees, and (depending on where you are) there are further government grants/tax-breaks (at least this is so in Canada).
You would need to find some Universities with strong co-op programs though.