Accessibility
Of late, I've seen a lot of developers make more effort towards accessible web development. While this is not a new thing, it is gaining popularity among many developers who previously either weren't aware of the ideology behind it, or the legal implications it can have.
Accessibility has long been a subject which I've looked at, mainly because I believe in the web as a universal medium which can and should be available to everyone. This is partly the reason why my choice in career has taken me down a more server-side development route than client-side, working on complex PHP systems instead of JavaScript or Flash ones.
A large part of testing accessibility is a manual effort, like checking there are valid alternatives to audio-visual content and not just a 'sorry, you need the latest name of media player to view this content.' and making sure that content flows correctly when styles are turned off (how is software to know what you intended with the content?) This process adds so much time to development, that I can understand why so many people have been slightly reluctant to create code with this in mind.
I've been looking around for a while for decent online tests, as a means to help automate some of this work, but the best I found that was free to use was Electrums Web Site Accessibility Testing Tool. It's a fairly decent tool, although the free version is pretty sparse on detailed information on what it considers errors on the site. Still at a disadvantage, I considered how much work would be entailed to create a simple testing suite myself, when I came across the Firefox Accessibility Extension, which has proved to be, I think, as essential an add-on as Firebug is for all web developers. The only drawback with it I notice so far is that it says <h1> tag should match all or part of the <title> of the page; that is, the entire heading needs to exist as-is within the title. This is not something I'd have considered a major issue, and up until now, I'd not even heard about it, despite reading over various accessibility guidelines.