Networking is a rich source of potential co-founders, partners, employees and so forth. It's a chance to get to know like-minded people and for them to get to know you. It takes time and patience but can be worth it. As babul pointed out, go where the types of person you need hang out and learn to speak their language.<p>Advertising for a co-founder is another way to do it, but I have always thought that makes for a more formal business arrangement, rather than two (or more) people united by a passion for an idea. Perhaps I'm romanticising slightly, but if you advertise for a co-founder I think you will need to spend more time (and money?) on formulating a pretty tight Job Spec to make sure all requirements, dependencies etc are watertight. This may not actually be a bad thing, but you should be aware of it, all the same.
As far as websites go younoodle.com is best one I have seen so far.<p>If you are at/around a uni look for a suitable society e.g. BASES at Stanford, OE at Oxford, Imperial Entreprenuers at Imperial.<p>If you can, attend hacker meets especially events like barcamp and some of the (un)conferences put on.