I may have a couple guesses, but I'm pretty sure I'm wrong.
Domestic Shorthair Tabby
Thanks to @Stig Tore. But that says very little if any thing about your cat. Other than that it's a cat: domesticated from wild cats with a certain pattern to their fur.
Now, if you could take it to a cat show judge, you might get a better estimate. They do know their stuff better then some internet stanger(s).
In the end, if you don't know the breed, that is, you don't have the papers from the parents, it's not from a recognised breed. But papers or not, that has no influence on the suitability for being a good pet.