Is it possible to conduct effective marketing campaigns with free tools (freemium resources)?
I think the answer to this is yes, but I don't believe this can be your long-term strategy. Instead, this path should only be taken in order to prove the value of certain tools with little to no financial risk. Once value is proven, then the more complete, paid versions of these tools should be purchased and implemented (if they exist).
Free versions of apps or software are usually limited in some way. While prudent to explore before you fully commit, these resources could take a more skilled marketer to actually be more effective. It may require more thought and more time to determine how to use it without incurring a cost...this could end up costing you more in the long-run.
There are some exceptions to this general rule though, and one of those exceptions is the HubSpot CRM. It is one of the more complete, yet simple CRM tools I have worked with and it actually is free.
Here's a blog post from my friends at Weidert Group from 2013 (so it's slightly dated, yet still relevant) highlighting 8 Freemium Tools for Inbound Marketing Users. I would add a few to their list: MailChimp, Buffer and IFTTT.
Here's a link to HubSpot's free offering. I've yet to use Marketing FREE, since I'm so accustomed to the full version I doubt I can live with the much more limited, free version even if it is only for a few clients. But I have tried and loved both Sales FREE and FREE CRM. Give them a shot and once you discover how wonderful they are, give us a shout and we'll help you move to the paid versions.