With so many different apps on the web its hard to know which marketplace to find suitable apps. Here is a good list of the various marketplaces available. Typical though depending on what brand of mobile phone you are using most recent phones will have a function allowing you to connect straight to their respective marketplace to download the selected app.

If your a software developer and wish to publish an app in the market place try Goggle's Android Market place or Apple's Market place for starters. Here are some good reasons for developing an app.