Sure it's possible!
I run my own, internal banner ads system, but a few months ago I joined Project Wonderful, which is much more easy for everyone, advertisers and publishers.
For advertisers, all you need to do is to open a free account, upload your banners, chose what site(s) you want to display your ads on, and you place your bid. Some sites (like Black Flag TV!) accept $0 bids, which means if no paid advertisers are bidding, your ads are being displayed for free. You can bid as low as 1¢ to secure your ad space, and from there it goes with the demand. Obviously, the more popular the site, the higher the minimum bids. It's an auction!
The cool thing with Project Wonderful is that its community is mainly around comics, manga, cosplay, conventions and indie stuff. And unlike Facebook or Twitter ads, you know exactly where you are displaying your ads, and you have access to all metrics of the publisher's pages.
Give it a shot! You can check my advertiser's profile here:
See you there!