This is a very good question with many different answers. I will vaguely touch on some of the different money making methods I have used if you would like more details on any one please feel free to contact me either in game or via yahoo.
…1)Non combat skills
~Fishing~ Sharks, and monk fish, Fishing trawler for Manta ray and sea turtle you can even “free load” although this is unpopular with other players.
~Hunting~ polar kebbits, carnivorous chinchompas (stackable)
~Farming~ Grow Marigolds, ranarr, snapdragon, spirit weed and toadflax.
~rune crafting~ is almost always profitable (You can kill Banshee's for noted pure essence a lot faster than mining for it ) If your not high enough for laws or natures there are worlds where you can run these for others.
~Cooking~ Tuna Potatoes, curry stew, and pizza’s
~Thieving~ cave goblin wires from Dorgesh-kaan.
~construction~ Make Teletabs via Lectern in player owned house.
~Mining~ coal is slow but pays very well. If you are high enough Adamintite and Runite.
…2) Using Combat
~Dragons using Melee or Range.
Green dragons in the Wildy or in the chaos tunnels,
Blue dragons in Edgeville dungeon, in ogre enclave and there is one in legends guild.
Black dragons with 70+ ranged or Kill them with protect from Melee (this cuts into profit as your using prayer potions though this can be minimalized by using prayer saving armor)
~Chaos druids, Unicorns, Moss giants, Vampires, White wolves, Cockroaches, Jungle spiders, Iban Disciples and Avanasis are all good monsters to fight for profitable drops.
*If you’re not interested in the prayer experience you can always kill and sell the bones. Ourg, Daganoth. Raurg, Fayrg and Dragon, sell best but even regular bones sell well.
** A fast way to big money is PKing but this is a risky endeavor. You can loose just as well as win. Remember there is always someone bigger and meaner than you... So you are liable to loose more money than you win. The more you risk the better your pay off its all on how gutsy you are.
…3) Slayer~ In general Slayer monsters pay well. Consider the food you use an investment. Most assignments drops more than pay for the food needed to fight them. The possibility of receiving a rare drop makes them even more enticing.
…4) Merchanting...watch the prices on the exchange then buy popular items when low and sell when high… There are several items you can buy in shops all over Runescape then sell for profit at the exchange, but as most of these are not stackable this can take several runs. Pineapples, seaweed, vials, bolts of cloth, Sacks, baskets, dyes, tinderboxes and Battlestaves are all examples of items that can be bought in stores all over Runescape and sold on the exchange for more.
… 5) There are several quests that are well worth the effort.After doing the Throne of Miscellania (and Royal trouble) quests,You can get your kingdom workers to gather resources that can be sold. And if you do the Hand in sand quest you can get free sand daily.
…6) You can collect Snape grass, Mort myre fungus, Planks, White berries, Potato cacti, Flax (Spin the flax into bowstrings for even more profit),Red spider eggs, Blue dragon scales, and Swamp tar. All these sell well.
…7)Clue scrolls are a roll of the dice but they have the possibility of large large payoffs! Clue scrolls which are dropped by certain monsters more often then others... (I kill Daganoth’s with a cannon to get mine fast ) .
… 8) You can hang out at the Falador drop room... I find wearing a ring of wealth helps (this is controversial)... it’s just up to luck what you can get and how fast you can get it.
…9) Diversions and distractions... Penguin hunting can make as much as 97.5k a week and Shooting stars can give you 106 k a day in runes coins and ore.
….The thing is the game is designed to take time (which is why my answer is so long).. Your suppose to have to work for it. Jagex does a good job of making sure there are no quick ways to fast cash! I answer this same question for a lot of people, often they want to know which one is the best method. There is no best method. They are all good solid ways to try out and figure which one YOU like the best (my personal favorite is chaos druids and slayer)... remember the point of the game is to have fun. So find the one you like, sit back relax and enjoy!