I take it your talking about one of these chaps.
If so they eat woodlouse,flies,centipedes,midgies,grubs and mites basically anything they can trap and kill. It's called the Tegenaria duellica or Giant house spider and it's closely related to the funnel web spider. Although it's bite is toxic to it's prey your quite safe, they can be known to cause great alarm due to their speed in fact they where in the guiness book for a while according to the Wiki
I never kill these fellers even when the cheeky bleeders come strolling across the floor like they own the place. They are in fact prolific hunters and that gargantuan sized chap in your house is so big due to it's efficiency at trapping and killing almost anything it can get it's teeth into.
i get loads of these, the kids tease them, and if you annoy them enough, they will jump at you. not long ago i got bitten by one on the side of my knee, and it swelled up like a baloon for about 3 days, so now i dont let the kids tease them any more, and i stomp them flat