I discovered Juce via the rather handy list on Wikipedia. It's actually really easy to get started with and provides lots of documentation for the burgeoning front end developer. And like any decent GUI toolkit it's cross compatible on Windows, Linux and Mac (and even iPhone).
And that's why I'm talking about it today. Where most GUI toolkits seek to wrap the native functionality of the platform or at least emulate it in some way, Juce has it's own look and feel (which is actually quite similar to OS X) which it maintains across all platforms.
This has the end result that (for the most part at least) an application developed with Juce will look the same no matter if it was run on Windows or Linux or OS X. This also means it won't blend in with your standard window decorations.
I'm in 2 minds about whether this is a good idea or not. On the one hand, variety is what keeps things new and interesting, and if you can pull it off well, like Google Chrome or Steam, you can get away with it. There's also the idea that if you have to use the program in an unfamiliar OS you won't succumb to culture shock doing so.
But on the other hand, consistency (across your platform of choice) with window decorations makes new programs easier to figure out and understand, and can also make them feel a lot more polished and professional. I've downloaded many applications from Sourceforge that are perfectly good at what they do, but because they've used a GUI toolkit that draws the widgets differently from the native application, it can stick out like a sore thumb. Take BoncEnc - it's a fantastic program that I use to rip my CDs for playing on my MP3 player, but it doesn't invoke confidence to look at. The strangely thin menu buttons (and drop down items), and the title bar with it's minutiae minimise/maximise/close buttons - it just feels so out of date.
Another thing that Juce does is adopt the OS X traffic light system for the close/minimise/maximise buttons. Unfortunately (in Windows at least, and probably Linux too) to stay consistent with the standard order of these buttons the colour order is amber->green->red. Also, in my personal opinion, I don't really consider "maximise window" to be the opposite of "close window". For me, it would be "Run", but that's not really applicable in this context. In the standard Windows XP theme, the close button is indeed red, but the other two are just blue, the same as the rest of the window decorations.