APIs have started to abound in financial services.

In the last few weeks, three important APIs, or application programming interfaces, have been launched from Capital One Financial Corp., Lemon, and LevelUp.

This matters because APIs serve as contact mechanisms – virtual handshakes, if you will – that allow various software to interact. The first API was released by eBay in 2005. Google was another early leader in opening its APIs to allow the creation of “mashup” applications. The location data in Google Maps is a good example of an application that has been used to create mashups.

Social media companies also developed APIs to allow programmers to connect to their data, and rich ecosystems of apps and ancillary products built up around Twitter and Facebook. Twitter has since started to limit the functionality of its APIs in order to exert more control over its virtual ecosystem.

According to Programmable Web, a blog that tracks APIs, there are nearly 300 financial and 190 payments APIs available today. A total of about 7,000 APIs are available to programmers today, according to Programmable Web.

Capital One’s API release was announced with some fanfare earlier this week. In the spring, the Virginia-based bank acquired mobile startup Bankons, and converted the operation into Capital One Labs, from which springs this API. The API will make Cap One’s deals, rewards, and user identities available for developers to offer personalized coupons and offers for their customers.

“We know we won’t always reach the customer where and when they’re shopping, but if you can, we should talk,” Capital One’s Paul Sun said.

Mobile wallet solution Lemon released its API last June, but made a splashy announcement about it yesterday.  Using the Lemon Application Development Environment (nicknamed, you guessed it, Lemonade) developers can add loyalty, payment or other functionality to the app, under their own brands. The Lemonade Stand will showcase the Lemon app’s partners. To add a participating card to your Lemon account, customers no longer need to take a photo of it. Now they can search for it on the stand. Very cool.

LevelUp’s API was last updated in March, and last Wednesday the no-fee mobile payment company added several new POS solutions to its ecosystem.  LevelUp has been busy. Last week, the company announced a white-label version of its product to allow merchants to brand their own solutions. LevelUp claims 300,000 users and 4,000 participating merchants.