I have not tried these all yet, but they're worth a look.
Google Mobile brings the ubiquitous Google products like Google search, Reader, Maps, Gmail, YouTube, Photos and many more to your mobile phone. It supports almost all the multimedia phones available including the iPhone.
Shozu connects your online life to your mobile phone. It supports a huge number of social sites which include Flickr, Facebook, Twitter, Blogger and many more. You can upload photos and videos, download friend's feeds, read and reply to comments and a lot more right from your cellphone.
Mini is an awesome free-to-download mobile internet browser that lets you browse the web in a similar way to your computer's browser. Instead of showing the entire page on your little screen, it cleverly presents an overview of a page and you can easily zoom in and out.
Zyb is an online service whose primary task is to safely backup your contacts on the cellphone. Apart from that it also lets you sync your contacts and other things, share media files and chat with your Zyb friends online.
Qik is a cool app which lets you stream live video to the web from your cellphone. I haven't tried this yet but it certainly looks like an easy process without the need to do heavy downloading or installing.
Fring is an easy to download and install mobile app which lets you make free calls and chat to your IM buddies on Skype, MSN, Yahoo, Gtalk and many other services using your cellphone's internet connection thus saving you money on outgoing calls.
Frucall helps you to compare prices of a product at various online stores like Amazon, Shopping.com etc and hence helps you to clear your doubts about the cost of a particular product and where you can find it at cheaper rates.
Buxfer helps you track your expenses on your mobile and lets you know if you are going over budget.
Mobiluck lets you stay in touch with your friends and alerts you if they are nearby. It also lets you chat with them for free.
Wapedia is a one click mobile encyclopedia which lets you search various Wikipedia entries and also search for images. A good app for students and researchers on the go.