1. Pandora - This app turns your phone into its own disk jockey. I own a cheap pair of speakers and I can plug my phone into them through the auxiliary port. Admittedly, if you listen to the same stations enough, you start to hear many similar songs but Pandora streams high quality music right to your phone for free, cannot beat that!
2. FlashLight - You have no idea how many times you will use the app, especially if you have a roommate. It is surprisingly bright and allows you to easily see in front of your phone without lighting up the entire room to bother your sleeping roommate. It is also free so if you don't like it, you can delete it without any problem.
3. TaskKiller - I will be reviewing this app soon but just know that it speeds up your phone, a lot. It stops processes in your phone that cause extra memory uses and takes a huge load off of the battery, adding hours to your battery life. It is also free and can be set to stop processes every time you turn your phone off.
4. Handcent - This app gives you a break from the default text messaging service. It adds the ability to reply to text messages without unlocking your phone or quitting other applications, an option that is life saving for anyone who is a heavy texter. It also allows you to customize the look of your messaging service. It is totally free and has many options.
This list is just a general impression of some great apps, I have over a hundred apps on my phone, many of which I use nearly daily. I simply think that these apps are the most useful apps in general. If anyone has a suggestion on apps or a comment about the apps that I've mentioned in this post, feel free to comment below and be sure to follow for more great reviews and ideas.