Searching for a programmable microcontroller (USB & LED dimming)
Arduino IS based on a microcontroller (by atmel) and is basically a breakout board for such microcontroller plus some friendly way to upload a program to it.

It's programmable in c++ so it's quite easy and, as @Michael said, the community is huge. You can use the program on a bare chip without the board if you provide the proper connections, parts etc... 

That said i'm a PIC user and i don't really like the bulkiness of arduino (both in terms of price, physical space and memory occupied by the bootloader) and the way its community works. 

BUT if you want to program a pic or an avr you need an external not-so-cheap programmer (search for pickit2 or avr dragon...way more than an arduino) and a proper c compiler. Then remember that usb access comes at a price (i.e. writing a program to use it but then you need a way to program the chip in the first place!). 

Arduino has everything you need to start FAST and such a program would be nearly trivial with all the libraries, a thing i wouldn't dare to say for a "from scratch" project like yours. Then again, if you want to learn bare embedded programming just grab a micro, buy or build a programmer, download the tools and start by blinking a LED then build from that. I'd advocate for PICs but really it doesn't matter. AVRs have nice open source tools while PICs are slightly cheaper. You'll find hundreds, thousands of discussions on which is best and why they're wrong :P