Excellent Web to Lead article
Sample PHP Script
Sample Code to give Web Developer
The above cURL code for PHP will work for WordPress of course, but also see the articles shared on this site for WordPress.
See this app Salesforce Suite | Drupal.org
I would not recommend this. See my blog post on Using Wufoo Forms with Salesforce.
API vs Web to Lead
There is some talk about that the Web to Lead is not the best way of doing things, but for small volumes I think it is fine. The only other option is to directly interact with the Salesforce APi, or use a product like Form Assembly.
It is just not feasible to use the API to simply create leads. Using the API would require that the Website contact forms are https:// (which means the organisation purchasing and maintaining an SSL Security Certificate), and using a whole extra Salesforce licence to give the API permissions to only write to the Salesforce Leads object and not read all of the data in Salesforce. (At $180/month, I'm am sure that is not something that the org will be willing to pay for). Apparently Salesforce Identity will allow you to use SFDC as the Auth provider for only $1-5/m but I'm not sure technically if or how that would work. See Post in ButtonClick Admins.
Articles linked from this site about Web to Lead