Heres two websites you may of already found. They will have better information than what I can provide.

http://www.cyanogenmod.com/
This is a popular alternate ROM (I use it) for alot of Android devices. Its not even close to the only alternate ROM, theirs tons out there.

http://www.xda-developers.com/
This is a great website for information. All kinds of stuff in here.

I use two markets. The Google Market and Amazons Market. Amazon gives you a free paid app every day, its pretty damn nice.

Being able to use the phone as a hotspot anywhere is beyond handy. I use this function almost everyday. Even better its free with an aftermarket ROM and even free with some phones/vendors with a stock ROM.

XDA would probably be the best place for you to start reading up on what the possibilities are.