Here's another short an succinct list of Popup systems we use and love.
Popups work. Hands down, full stop, etc. They work on load, on exit, on click. They are helpful and consistently bring new subscribers to people's lists. If you're not using these on your site yet, please consider them.
Layered Popups
These guys have renamed themselves Green popups recently, but they rock and are super affordable, with a one-time $24 fee that will get you professional results for very little. I've used them with my own sites and clients for YEARS. I mean that literally.
They integrate with just about every Mailing List system you can think of, with customizations like adding tags, etc. to your contacts, so I fell in love with them years ago and continue to use them when appropriate (you can make quick popup based on clicks, etc.)
This is a WordPress installed system, so if you run many sites, you'll need to install LP individually on each system; if you have to manage many, and this may be cumbersome for you, and you should check out Optin Monster for a one-stop location for many sites.
Overall, the visibility and load time is totally consistent; it's very light on load time, and with its awesome drag/drop editor, you can either start with a template or build your own from scratch, which will make the newbie and the CSS pro happy.
Optin Monster
I like these guys a lot, if for no other reason but that unlike Layered Popups or Thrive Leads, you control and make your popups on one site. This is good for agencies like us, having to manage certain systems for clients, and we have everything, including statistics, all in one place.
This is a monthly/annual service, so not a one-time purchase like Layered Popups, but worth it for the ease of use, as they have nice customization and options and also integrate with just about every service you can think of.
We have seen OM work a bit wonky at times, when it hits adblockers or Brave browser, so try them out and see if they work for you.
Another PRO is that you don't need to have a WordPress site to use them. You can create a popup and embed it anywhere. You can also create a stand-alone link that you send people to just to collect leads. You literally don't need a site to use this service.
I'm not sure if they integrate with plug-n-play systems like Squarespace, but those guys usually have their own built-in systems.
Thrive Leads
I really love Thrive Leads, which integrates beautifully with your Thrive Theme if you use it (a WordPress system), but can be installed for other WordPress themes as well.
It is a bit more complex (and robust) than Layered Popups, in that you can create sets, shortcodes, conditions, and many many customizations. They also have a TON of designs.
That said, for beginners, it might be a bit overwhelming.
So definitely think about EASE first if you need that.