Mashup is a d20-based homebrew ruleset built up over the years to play a very particular sort of game. It's flexible in the extreme, if a little (okay, a lot) unbalanced at times. You can build anything you can think of, using a fairly concise set of powers. It's class-based, but only loosely, and most of your abilities are purchased with character points allotted for character creation and earned in play.

Mashup started as a combination of Wizards' d20 Modern and Guardians of Order's BESM d20, modernized a bit with Paizo's Pathfinder RPG. Eventually, it outgrew just being a mashup of three rulesets (hence the name), and the 2.0 version is largely independent of another ruleset. I say "largely" because rewriting the entirety of a d20 system's combat rules is a very special hell and I've not finished that journey just yet.

You can find everything you need for character creation in the Mashup 2.0 Character Creation Draft, save equipment. That, you can find on the NowhereMash wiki. You can use other d20 products. In fact, you're encouraged to. There's a conversion guide in the "what you need" thread.