So, according to Anonymous (in the comments section of 'Script Register', below), InkTip is a very good website that gives aspiring screenwriters genuine opportunities (and doesn't charge a fortune for the privilege). However, reading between the lines of Anon's experience, it shows you need the same amount of diligence and talent as you do when forging contacts in the real world. I think there may be a tendency for some wannabes to look at these screenwriting sites as definite opportunities, especially after they've forked out some cash to get insider access to the site. It boils down to the same thing: you've got to be good, and your stuff needs to be thoroughly prepared and well-written. Anything else, and you're just another stat on the spec pile.
A few years ago, I signed up to Script P.I.M.P. which is described as: "a community based research tool designed for writers & film industry professionals." It's a neat site but while you do get access to production company details, it has a layered system of access where, by and large, you need to be recommended to the companies by Script P.I.M.P. itself. And the only way to get a recommendation is to submit your script for 'coverage', and, naturally, this comes with a fee attached (quite pricey, too!). Still, it does have some useful features - a Writers' Database, a Writers' Workshop and a Screenwriting Competition - but in truth, I don't use it at all.
In the UK, you have, of course, Shooting People (£30), but that's more of a filmmaking community site rather than offering a platform for various opportunities. They do have the pitching bulletin each week (anybody get anything from that?) and you can easily find crew if you don't have any contacts, and that's certainly useful. The regular bulletins are filled with news, comments and opinion, so it's a decent site if you just want to keep up-to-date with stuff, and feel part of a wider crowd who understand your pain.
Similarly, there's UK Screen (£15), Talent Circle (free) and the quite handy Mandy. I was a member of UK Screen once but like Script P.I.M.P., I've kind of forgotten about it.
Generally, these types of sites are fine for networking and getting to know the scene. Don't view them as potential short cuts to success or a back door into the industry. Use them for what they offer but the rest is down to you, your writing and your talent. Nothing else.