Item lending.

Lend out any high value items you have (In the norther part of the GE, World 2, members only) I usealy rent out my Saradom Sword for 350k for 12-16 hours. It takes about 15mins and is super easy.

Other items to lend out for a good profit are (but not limited to):
1)Abby whip
2)Any partyhat (if you had one you wouldn't be here though)
3)Any rares (santa/h'ween/other partyhats you have)
4)Darkbow
5)God bows
6)Staff of light
7)Godswords
8)Dragon 2h
9)Dragon claws
10)High end Dragon armor (chain/plate/full helm)
11)Dragon resource equipment (Pickaxe and hatchet)
12)Godwars dungeon armor (bandos and armadyle)
13)Dagonoth rings (berzerker/archer/seers)
14) Theres tons of others but in tired of listing them.

Black Mask

Once you have 58 slayers, and access to Mos le'Harmless, you can kill cave horrors, they drop herb seeds and black mask (the black mask rate is more common than diamonds, and dragon equipment).

Once me and 2 friends went there for an hour, I at level 103 cb at the time, made a little over 2mill
My friend who had 117 cb made about 6mill.
My friend who had 79 cb made about 400k from seeds.

Sharks

Cooking sharks can sometimes make money, depending on market prices, found here:

http://services.runescape.com/m=itemdb_rs/results.ws?query=shark&price=all&members=

IF the prices are good you can make up to 100k/hour (With great cooking xp)
IF the prices are bad you can loose up to 500k/hour

Cheese

Buy buckets of milk on the G.E. Take them to the cooking guild (must where a chef hat) and on the second floor turn them into cheese. Make all the wheels at once, then cut them all in the banks, the higher your cooking level the more money you make.

At 50 cooking i made about 100k/hour
At 80 cooking i made about 410k/hour

Chaos druids

Once you have level 10 dungeoneering, you may visit the chaos druid cave, kill them for ranarrs and other herbs,  you can make up to 750k/hour this way if done properly (Try using a amulet of glory to bank).

Snakes

After doing the Moss le' Harmless quest line, you can go north of the trouble brewing bank and kill snakes, (lvl 6 i think?) and take the hides, these sell for 3k each, more if you tan them.

WARNING: BRING ANTIPOISON.

The most ive made this way in about 350k/hour