1st post here. I was inspired by your rebound from the Panda penalty and when I saw the internet marketing area I couldn't help but jump in with some advice.
It's tempting to spend all of your time and energy on getting Google/Bing rankings, but as we've seen the engines can shift ranking factors in an instant and destroy years of hard work and sap profits from great websites, possibly even shutting down many.
As an internet marketer it's important that I mitigate the risk of exposure to such changes to my clients. That includes keeping things above board, playing nice, but also not holding onto Google like a needy ex-girlfriend.
Here are the different types of traffic sources and a summary of the strategy around them. Many you know, some you don't. Feel free to add more below.
PPC (search engines) - target search queries, steal eyeballs from SERPs. Send to stripped down landing page for pure conversion goodness (no distractions). Local business? Geotarget State/Region or Metro only! = Win
PPC (social) - target users with brand mentions and keywords. goal is awareness, less conversion-centric.
Retargeting (display) - target users recent search history with colorful ads that match old search queries. Send to stripped down landing page.
Other Display - target users based on assumed psychographic profile (i.e. moms on a mom site, gamers on a game site, etc..)
Sponsored Tweets (social) - never used it
Facebook - use page to update, a lot like a blog. new things, cool things. apps are becoming less prevalent on the site these days, more prevalent off the site (i.e. comment apps). Make and curate general interest pages, use to promote your site(s) where possible.
Twitter - use to update users, use as customer service channel, use to communicate with industry partners.
LinkedIn - find like minded pro's, join industry discussions, build brand equity with industry peers.
Digg/Reddit/etc.. - bookmark your neat/cool stuff, participate in conversations via comments, add value to the community via quirky stuff (images, videos). mixes will with content (farther down)
Related forums - post helpful friendly things, ask questions, answer questions, sell things, buy things, always append URL in signature or in body of text (if allowed).
QnA - answer questions, hopefully answer is found on your site. answer other questions anyways with answers elsewhere. brand equity boost, adds credibility.
Vertical Search - not google? not bing? works well in travel, real estate, automobile, computer parts and gadgets
Blogs (commenting) - comment only when something useful to say, add in link where applicable, not where it doesnt fit.
Deal sites - offer a deal. these things are everywhere right now.
Listing sites - yellow pages arn't dead just yet. get listed in as many as you can.
Viral Videos - funny, humorous, etc.. do your best here. they can't all be homeruns. find quirky people on fiverr.com
Informational Videos - answer questions (see QnA above) make a video answering commonly asked questions, showing off a product, showing uses how to use your website, etc..
Reputation Videos - tout customer reviews, testimonials, etc.. found on other websites.
Infographics - the internet loves these. do some research on a related, possibly hotbutton topic and have a graphic designer make an infographic.
Motion Infographic - see above, but a video set to music with moving images instead of one graphic.
Guest Blogs - Write on blogs that are popular, influential in your space. Tons of sites catering to this.
Manuals/Guides - write up manuals, instructions, etc.. and upload to sites like issuu.com (get a link too)
Slideshows - share the knowledge, like info videos but in a slideshow format. add links at the end for your site(s).
Whitepaper - do a case study, share what you learned and how you tackled an issue. typically in PDF format.
Donation - give some money to a charity and ask for a mention on their website. depending on the donation size they might be willing to promote your site in an email, etc.. to their members
Hire People - employment branding. offer an internship, etc.. and get mentions on job and school websites.
Lead a Tribe - check out seth godin's book "Tribes" great stuff. grab hold and become the leader of a segment of your industry and you'll get traffic from sources you never knew existed.
Help a Reporter - help a reporter with info on an upcoming piece, get a good mention. boosts traffic instantly, and adds credibility.
Mobile - make a helpful application that matches your industry
Sponsorships - give away stuff, get banners/links, mentions and traffic from it.
Contests - come up with a unique contest, pump it out over social media and watch the traffic fly in.
Affiliate - find other websites you can partner with, maybe they have something you need, and you have something they need? or maybe just pay them per sale/lead.
Newsletters - update current users on new things, include links and calls to action for what you want them to do in each segment.
Old Customers - do something crazy, make an insane offer or anything to try and 'wake the dead' in your email database.
Milestones - are you customers in it for the long haul? keep them apprised of their progress.
Related Notices - maybe hit customers with new stuff related to the old stuff they purchased/downloaded.
Tradeshows - attend any that you can
Bands - sponsor a local band. if their audience is in your demo get them to put a nice big banner up. good boost to brand awareness, some traffic, but brand stays in photos for years to come.
Shirts/Hats - give out clothing with your brand on it.
Flyers - works ok for local business, not so well for regional/national
Print Papers - yes there are still some in small communities. ad rates are typically a little higher than online, but a good way to reinforce online messages or to reach isolated groups.
Obviously search engines represent the single largest source of all of these types of traffic. However, boosting your brand awareness will drive your type in (direct) traffic. This for most websites is the largest source of traffic. Don't be scared to do things that keep your brand/website in the minds of your potential future consumers. Find a clever way to run a marketing campaign and combine various aspects of these traffic sources.
Always make sure you have a way to track the success or failure of a campaign
and follow this one simple rule "Don't FAIL, A.I.L." (always include link).