PubMed On Tap is an application for your iPhone, iPod Touch, and iPad that lets you search PubMed and PubMed Central while on the go.

Find, display, and import reference information and pdfs of the full article (if you have access privileges). You can search your personal reference library, organize your references in groups (static and smart), annotate your pdfs directly within PubMed On Tap, and email references from your device to yourself or others in a form suitable for reading or importing into desktop reference management applications. Many features are optimized for reference retrieval and management (advanced boolean searches, search for Full Text or Free Full text, recall recent searches, etc.).

Use iCloud to sync all your libraries and attachments across iOS devices, and have full access to your references and sources when you're on the go.

More search engines: Google Scholar, JSTOR, LIbrary of Congress, arXiv, and Web of Science can be found in Bookends for iOS.