SEO. It's free. Ohh wait..you want something where you can throw $50 at and make millions right? Hmm...you'll have better chances buying a gun and robbing a bank.
Like the poster above mentioned, SEO is free. So is article marketing, forum marketing. You can write and submit press relases for free (you have to know who to contact at media outlets though). You can "guest blog" on other popular blogs in your niche. You can contact other business owners who sell related products and services and ask if they'd be interested in joint venturing with you in exchange for a cut of the profits that result from the partnership - which isn't exactly free, but you wouldn't have to put up money in advance since you'd only be sharing a percentage of the profits from a successful JV.
I definitely have to agree with this. You want users coming to your site that are targeted -> that means they know why they are going to your site. The best and cheapest way to get a ton of real users, all of whom are targeted, is to have content that is right for social media websites. That's what MagicShovels does -> we promote content on social media websites like Digg which brings in tens of thousands of visitors to our clients' websites. Besides that, good SEO is something that will do it. It's something you'll only spend money on once (for the most part), and forever you'll have people coming to your website as a result of ranking high for your keywords.
If you can master the art of SEO and get a top 10 ranking in Google, then all your traffic is free, and your expenses become pretty much non-existent.