Developed web 2.0 websites can be a good source of free backlinks that you don't have to ask people for. Nonetheless, their development will take either hard work, money, or both. Anyhow, you can easily find a massive list of them in a any search engine search.
Of course, it is easy to find a list of web 2.0 on the internet but to be honest backlinks from Web 2.0 are really weak and it doesn't help more in SEO rankings.
I would suggest you focus on guest blog posts or building a list of quality sites and getting backlinks from them. That can consume more time and money but it will bring quality to your site when building backlinks.