If you're a London based web developer, or work in any internet related job, in London, there are plenty of regular meet ups, where you can meet people in similar jobs and even get free food and/or drink. Meetup.com is a good place to find them, but here is a list of some of the top ones I've been to:
Minibar - 4th Friday of every month.
This is a good place to get a free beer, or wine, while finding out about new web startups and meeting fellow internet professionals. The format is informal networking, followed by 3 or 4 presentations, one from the sponsor and the rest from new starts-ups wanting to showcase their business, then more informal networking. The sponsor puts money behind the bar, but it runs out pretty quickly, so you need to arrive early to get a free drink.
Flag and Bell Pub Crawl - 1st Tuesday of every month.
Originally set up by the founders of Trexy.com, this used to be sponsored and take the format of a quick talk from the sponsor, followed by informal networking. two pubs would be visited and everyone would get at least one free drink in each pub. Now, some of the regulars take it in turns to organise it each months, and it isn't often sponsored, so there's usually no free drinks or talk, but it is a good social night.
Facebook Developer Garage - monthly
As the name suggests, this is a meet up for Facebook Developers, whether you actually develop apps, or just set up pages and adverts etc, for your company. Tickets are £5-£14, but include beer and pizza. It takes the format beer, pizza and informal networking, followed my talks and presentations, the optional further informal networking in the pub afterwards. There's an interval during the talks, where you can get more beer and pizza.
London Bloggers Meetup - 1st Tuesday of every month.
A sponsored meetup for bloggers, that usually has free drinks. It takes the format on informal networking, followed by presentations from the sponsor and/or bloggers giving tips, followed by more informal networking. Anyone who blogs on any topic is welcome.
There are loads more. Some of them, I've attended in the past, but not recently, so the format may have changed and some I haven't attended yet. As I attend more and become familiar with the format, I will add them. In the meantime, feel free to add your own, in the comments.