When people think of tools, they think speciality items such as jeweller's tools, these could be useful but are very expensive. Some "hobbyists" or professional builder's buy/use lathes, drill presses, printers suitable for making their own decals etc. If you have the money and space, go for it, if not then read on.
If you look around your house I'm sure you'll find many useful items to help you in your modelling tasks.
Items around the house could consist of things such as clothes pegs, rubber bands, ruler, pencil, old cardboard, paper, paper towels (toilet paper), tooth picks, butter containers (for storing parts), tampons (for soaking up spills), nail polish (for painting), the list goes on and on.
Next time you're out shopping stop by the $2 shop (discount shop) and look for things such as a small set of pliers, side cutters, small files, sandpaper, screw drivers, tape, plastic containers all these and more can be found at flea markets as well.
Want to get more advantage but still stay on a limited budget?
In rubbish collections (inorganic or flea markets, not rubbish bins) look out for an old oven range hood (get checked by certified electrician before using) to make a spray booth, some old wood, again for the spray booth, old bits of angle iron (cleaned up to use for centering wheels), go to a florist and ask for some of that stuff they stick flowers into (usually green foam stuff) great for holding parts while being assembled or painted.
Coat hangers can make great stands for painting or assembling, old telephone wire (really a little big but can be used for engine wiring etc), plastic bags for holding/storing parts, solder (all thicknesses can be used), small drill bits.
If you can't afford a decent two-way airbrush use spray cans, check out places like the warehouse (similar to Americas Wal-Mart). Usually you'll get 300-500 ml cans for about $4 NZ, way better than $13 NZ for the 100ml Tamiya cans. Or if you want to get a bit adventurous check out your local paint store, they can fill a spray can for you (of any colour) in lacquer for about $20, painted in several light coats can last for several models.
The film canisters you get when you buy a roll of photo film, is useful for storing small items as well as making things. I haven't done it but reckon you could use them to make some rear wheel tubs.
Glue, thou the standard model glue these days is pretty good, if you want to use 2 part epoxies or super glues check out the discount stores.
Need more models but don't want to pay the earth for them? Check out your local paper or advertise for them, countries like NZ there isn't a big market so you don't have to pay collector prices. Also think about casting (or getting cast) your own parts. Having said this you should always support your local hobby shop so as not to kill the great hobby of modelling.
Need more help, hints etc? Check out the web, heaps of model related web sites and new groups, most are used as gossip columns but can still pick up great information.