Feeds Tamper is an add-on module for Feeds that lets you modify data before it gets saved. It provides numerous plugins to transform the data that Feeds pulls in. See the full list of plugins. Creating new plugins is easy, see the developer docs.
Enable the modules Feeds Tamper and Feeds Tamper Admin UI. Next, configure the plugins you want to use to modify Feeds data before it gets saved. Each Feeds import mapping can be manipulated individually.
Once Feeds Tamper is enabled, the list of available Feeds importers (admin/build/feeds) displays a "Tamper" configuration link for each feed.
Visit the mapping page for your import mapper (admin/build/feeds/edit/[your-feed-importer]/mapping and click on "Configure Feeds Tamper", or visit admin/build/feeds/tamper/list/[your-feed-importer].
For each mapped field, click "Add plugin" to enable one of the available plugins for that mapped field. Select the plugin to add from the available plugins drop-down, select your desired configuration options for the plugin, and click "Add".