MathML browser extensions
It's really annoying that there still isn't universal support for MathML. Things have got better recently though, with Safari finally supporting it. Of the major browsers, though, there are still two that don't.
First, IE. It doesn't matter. It is only used for three things: first, to download a browser that doesn't suck; second, if it is chosen by someone who is stupid, ignorant or malicious; third, by people who need to compare browsers.
Second, Chrome. Despite its lack of MathML, Chrome is IMO the best browser out there. It works on multiple platforms, has lots of extensions available, and has very good support for synchronising things like bookmarks and open pages across devices.
And now there's a plugin for it to support MathML! And of course Chrome supports it without a plugin on iOS, because it uses the Safari rendering engine there.
update: Safari on the desktop still doesn't render it particularly well, despite it working just fine on my phone.