Potions and Magic - I've usually had a very small local market (usually one hedgewitch or the like)
I also allow the PCs to pay sages and such for identifying magic items plundered from the dungeon. This also tends to be expensive, usually costing 50 gp or so to identify a potion and 300 gp for weapons and armour.
Fates Worse Than Death - catch a nasty disease from the giant rats? Or get cursed by the witch? Getting that sort of thing undone can cost some serious coin. The typical price I've seen for having a spell cast for you is 100 gp per level of the spell, making cure disease and remove curse cost 300 gp for each casting.
Throwing Money at Problems - Allow the players to solve some problems with money. Let them hire and outfit henchmen to accompany them on their adventures. A sage (2,000 gp per month) might be able to learn more about the dungeon or the evil duke who is threatening the region, while a spy (500+ gp per mission) might be able to ferret out the Duke's vile plans. Maybe the orc tribe will take a bribe to go pillage elsewhere, or could be hired to help take on the hobgoblins next door. Maybe the dragon won't eat you if it let it eat your horses.
Making Friends and Influencing People - Being known as philanthropists and high-rollers can result in beneficial modifiers to local reaction check rolls. This can include things like sacrifices at the local temple of a patron deity, weregeld paid to the families of henchmen who died on the last adventure, or rebuilding the orphanage burned down by the goblin lackeys of the evil duke. My college crew celebrated important milestones and achieving long-term goals with wild parties, in which they invited many of the important NPCs from past adventures. These were fun to RP, and allowed me to sow the seeds of future adventures. And, of course, they required the spending of lots of coin on food, entertainment, and clothes.
And if you're knighted after rescuing the count's daughter, you'll owe him a certain amount of military service every year. To avoid having dull patrols and sentry duty interfering with far more profitable dungeoneering, pay enough
Property - There's no need to wait until reaching "name level" before allowing the PCs to start spending money on lands and property. A small house in town can serve as a start, with a few servants and guards to protect it while they are away on adventure.
The nice thing about most of these suggestions is that they don't make the PCs feel like they are being punished for their success. Taxes and theft only make the players suspicious and angry. They can be used, but only with moderation. Instead, let the players use that money to make the lives of their PCs more fun and comfortable. Once you get the ball rolling, the players are likely to make suggestions of their own. Whenever possible, let them get what they want; "no" just shuts things down, but "yes, and..." creates new adventures and new fun.
Art by Joseph Mallord William Turner, Jean Limbourg, and Hans Makart .