Depending on what platform you're using, you can use Expression Blend to edit your GUI controls in WPF (.NET 3.5 SP1) to look however you want.
Otherwise there's always Photoshop. :)
The best image editor to achieve these effects is probably Adobe Fireworks, which definitely doesn't come under the freeware category at around £700 )and probably around $700 as well, although I haven't checked).
The best freeware editor in my opinion would be Paint.net.
However, I also think that you should seriously consider using the default UI components where available, as using non-standard widgets can significantly reduce the usability of your product.
I use Inkscape - it now has much more flexible control over gradients and opacity, which are key to creating good GUI elements.
We use Adobe illustator and Photoshop to create all of our Icons and Images, and when you know how to use it, you can make pretty good UI elements. Paint.net is as a freeware good for bitmaps and icons (a mini photoshop). but for icons i would recomend axialis icon workshop
Unless you have a budget, I like free so Inkscape is my vote too. There are plenty of tutorials on the web to help and once you learn a couple of usability points with it, it's a pretty easy application to use. Being vector based has benefits too as you can change images other people make pretty easily.
I use Paintshop, mostly. I have a copy of Inkscape for whatever needs to scale well.