Leadpages (often called a 'landing page' or 'sales page') are single web pages with one call to action, whether that be to buy a product, download a product or subscribe to an email list etc.

One call to action that you measure, track and use to attract clients into your online world.

Having worked with a variety of businesses, I've had exposure to several different lead page platforms, including

Leadpages.net - one of the best for measuring conversions and integrating with other programs.
Optimise Press - has a variety of templates and can easily integrate into a membership site, however not simple to use if you're technically challenged.
Ontrapages - great templates and forever free to use! (I currently use this for The Healthy VA lead pages)
Instapage - simple to use, flexible templates and they offer your first lead page for free.

Ontrapage is a great option as you get 10 lead pages forever free. It's also worth trying if you're looking to upgrade to Ontraport CRM in the future too.

However, if you aren't very tech savvy (and CRM tools aren't in the pipeline) I recommend Instapage as it's the simplest platform to use and you can still try-before-you-buy. Note: You need to upgrade to a paying membership after 30 days, however it's the most affordable option for low-budget businesses.