I've spoken before about building your customer avatar so you can connect with your audience on a deeper level. Essentially you build up a profile of what your average customer looks like so you can communicate more effectively. One of the activities we recommend is thinking about what other sites your customers might visit but beyond a few educated guesses and actually asking them getting real data isn't always easy since sites have become more privacy/security conscious.
However there is a little known trick you can use to get a general idea of some of the sites your customers are visiting. You start out by creating an account with Perfect Audience for retargeting campaigns (which are also very powerful in themselves). Then once you've gathered at least 1000 visits to your site with the tags in place, your ads will start appearing on their ad network. Eventually a profile will start to develop of the top 100 sites that your ads are served on and where conversions are coming from. This is usually used for optimising your conversions depending on the traffic source but it can also give you a window into some of the other sites your audience are visiting.
A brief look into the history of domains that appear in my list show dating sites, which could suggest that some of my audience are both seeking a partner. Several other sites are dedicated to CSS (a design oriented programming language), as well as a few icon and graphics sites which would suggest that there are a few web developers or designers visiting my sites. There are consumer focused real estate sites, suggesting that some of my audience may be in the market for a house, as well as a few hits on holiday accommodation and hostel sites suggesting travellers in the mix. There are some professional advertising blogs which suggest an interest in mainstream advertising and a few sites that seem so new it is possible the owners themselves or one of their close contacts are viewing the sites.
You can guesstimate a lot of things by looking at the sites that appear in your reports, but it is important to note that it is definitely not conclusive and should only be considered as part of further research. There is every possibility some of the sites that appeared in the list were launched by popups or over-aggressive marketing from other ad networks. It's also possible that other users of the same computer were visiting sites after the original visitor was cookied. However over time there is a high likelihood that you will start to see patterns and larger numbers of visits for certain types of sites so there is a fair chance these are things that your audience are actually interested in.