I've bought every house in the game except this one. I've had two different characters, one in the Thieves Guild, one not, and neither of them have had the option to buy the house. I think my first character was the Thane of Riften, but I'm not sure. I know my current character isn't though. What do I need to do to get to buy this last house?

You need to go through the usual 'help citizens of said "city" quests'. On 1.5 I started by helping Wujeeta (correct name?) down at the fishery. After that I helped an in-love Argonian in the bee and barb, then I helped Balimund with his forge. Didn't do any radiant 'kill X and return to Anuriel' quests.

Edit: grammar

Sort of.....

You start out the tangent quest for the Jarl with Wujeeta down on the docks. She'll ask for a healing potion to cure her... issues. Once complete, make sure to ask her where she gets her skooma which will start you down the line to a thaneship. And yes, you still need to help 5/5 but there are so many to help in town, this shouldn't be an issue... alchemist shop there's two, out of the city walls at the near by farm is one, the stables is another, the meat lady in the market, the blacksmith, the maid at the bunkhouse, the lexicon quest, all of those would qualify.

the tasks have to be for the citizens of Riftin, not the steward (I think). I found 60 plants for Maven's cute daughter, 10 fire salts for the black smith, three flawless gems for an inlove argonian. Donated to the temple of Mara and got married (if that counted I am not sure) I also helped that completely lost her marbles mage in the palace. I also convinced Sapphire to drop the money owed by a stable boy. And I think I also may have taken a letter to someone in Markarth for a fellow in the tavern.

Any favour done for any citizen in the hold will do, including the sale of a cabbage to a farmer or the sale of an iron ingot to the crazed miners of Shor's Stone who believe that they are mining iron in their ebony mine.

The Rift doesn't just consist of Riften. It also has Ivarsted, Shor's Stone, one mill, and 2 or 3 farms (is the Sarethi Farm in the Rift? I am not sure). Oh, and an orc stronghold as well. Finding enough people to do favours for should really not be an issue at all.

These favours aside, all you need to do is:
- give Wujeeta a healing potion. Inquire about her addiction.
- go to the Jarl of Riften and talk with her about the skooma dealer to get the key.
- take out the skooma dealer, stripsearch the house and find the note in the basemant.
- read the note.
- report back to the Jarl and be sent off to take out that base of skooma operations.
- kill some bandits in that cave.
- report back to the Jarl.

At this point, I usually still haven't done all of the favours yet, so she doesn't say anymore about the thaneship yet. As soon as I turned in the 5th favour though, she will have another line and offer the thaneship, which will be yours on the condition that you own a house in town.

Go buy the house from the steward.

Report back to the Jarl and all should be good and well.

Ive done all this and have no other options for the steward. What am I doing wrong?

In Lytha's post, the "read the note" part can be tricky. It must be in your inventory when you read it before you can move on. You can't just find it and read it......I don't think. I was hung up at that point for a while and then finally read the note while it was actually in my inventory and then I got the option to move forward.

Perhaps I'm mistaken and I just encountered a bug, but I feel like that was an issue along the way. I finally purchased Honeyside and it is my main house.....though, I only have 2.

Good luck! Honeyside is a cool house.

Lytha pretty much spelled it out, and take note of what Jabberdave said. Here's the UESP page for it: http://www.uesp.net/wiki/Skyrim:I_Done_Got_Thaned!

I ended up doing one extra "help the cits" quest/favor something didn't go through right. Not sure which one, so I did end up doing an extra. Try that if you think you've done the required 5 and it hasn't worked.

I'll bet that this is the issue. With everything else done, it has to be. Either that or a bug has struck; hopefully not.