Here is a list of the top apps you can get from Cydia
One of the first apps you should download from Cydia is RockApp. It allows access to the same apps you can get from Cydia, just with a more user-friendly interface 
Winterboard is an absolute must for any jail-broken iPhone. All custom themes are built to be used with Winterboard. This app is what you use to enable and set your themes on your phone
The ModMyi app is an essential if you want to learn what you can actually do with your jail-broken iPhone. It is a full featured forum browser for the ModMyi.com forums. This is where you will learn about everything from themes to OpenSSH to file mods, etc.
zToggle is a great little app if you are on a 3G. What it does is allows you the ability to turn Multi-tasking and Home screen wallpapers on and off right from your device, without having to go through the jailbreak process again
Sbsettings (Spring Board Settings) is another essential app to install after you jailbreak. What it does is gives you quick access to settings like bluetooth, WiFi, location, brightness, SSH, Processes, etc. It is customizable to most any setting you want.
iFile is a very neat app. It gives you root access to any and all files on your phone, right there on your device. No need to go through a WiFi connection. Just open iFile, browse to the folder or file you need. It also allows for editing of files on your iPhone. USE WITH CAUTION
iBlank is an important addition if you will be using custom themes. It puts "blank" icons on your home pages, effectively giving you the freedom to place your icons wherever you would like. This allows for the ultimate customization and also gives the space needed for those cool weather widgets
OpenSSH is a very important program to put on any jailbroken iPhone. It won't have an icon on your device. It is an application that runs in the background. OpenSSH allows the user (that's you) full "root" access to the system files of the phone, either through iFile above or through a WiFi connection using a program such as WinSCP. Again, USE WITH CAUTION. Here is a great walkthrough on how to log onto the iPhone and use the program (known as SSH'ing)
